A/N: This is another one of those stories that wrote themselves out of nowhere. I probably owe the entire Sherlock fandom an apology for this, but I've seen merfolk AUs for other fandoms before and the idea of doing a Sherlock one wouldn't let me alone. Johnlock, some smut, and general absurdity on the author's part.

"Whoa!"

"Watch its tail!"

"Hell's bells, it's big."

A moment of silence.

"Someone go get the Captain."

In his cabin below decks, the ship's doctor closed his laptop with a sigh. There were only so many times one could write "Day _ on the freezer trawler; nothing to report," on his blog before he would lose the precious few followers he had. They were on the sixth day of the outing already, and the absence of excitement was maddening. All the more irritating was the noise from the deck - the shouting had started ten minutes ago, and the First Mate, a woman named Donovan, had called for an extra net to be brought up from the cargo hold post-haste.

John stood and stretched before exiting his small cabin. The hallway was lined on both sides by sleeping quarters for the crew; further down the hall to the left was the hold, and beyond that the immense freezer which partly processed and froze the ship's catch for their week-long excursion. Immediately to John's right were the narrow stairs which led up to the deck. The doctor ascended these, and found that the crew were all gathered in a huddle around something lying on the cold metal floor.

Captain Lestrade stood, and, seeing John, beckoned him over.

"Ah, Dr. Watson," he said. "Just the person I wanted to see. Come have a look at this, and give us your professional opinion."

John shouldered his way through the crowd with murmured "pardon me"s and "sod off, if you wouldn't mind" until he got to the head of the group. Then he looked down and gasped.

Tangled in a red nylon net was the strangest creature the doctor had ever seen. It was almost a man, with pale skin and a mess of night-black curls, but there were little scales pockmarking the skin of its forearms, and there was a flap of greenish webbing between each long finger. Its chest was bruised and bloodied from struggling against the netting, and John might have felt pity for it had he not been so much in shock. At its waist, curving hipbones blended into silver scales, and below that, where legs should rightly have been, was a fish tail, slick with seawater and glistening the steel grey color of the ocean after a storm.

"What is it?" John asked, bending over to look more closely at it. Its eyes were closed, and its breathing was shallow. Whatever it was, the doctor realized, the fight with the net had exhausted it.

"A siren," the Captain said grimly.

John's eyes widened and he shuffled backwards a pace.

"A siren?" he repeated nervously. "Should we have it on board?"

"That's the only thing to do," Donovan interjected, sneering at their captive. "Throw it back in the water and it'll drown us all. They're psychopaths. Anderson!" The skipper looked up at her, blushing slightly. "Help me take this thing down to the cargo hold. We've got a tank in there that'll keep it until we get back to England. There's a Queenly reward for catching a siren, boys, make no mistake."

"Doctor!" said the Captain. "You carry it down there and keep an eye on it for the rest of us. Donovan, you and Anderson can fill up the tank for Watson."

John was reasonably sure that the First Mate scoffed at him, but as he cautiously scooped the merman, net and all, up in his arms, he found he did not care. Ahead of him, Donovan and the skipper descended the stairs. John went to follow them, making his way to the cargo hold where the other two were already dumping a barrel of drinking water into a glass tank as long as a man and high as one's knees. The lid rested against the wall, but John considered the claustrophobic container and knew that if he shut the siren in it, he would have nightmares for a week.

The First Mate didn't even spare him a glance as she swept out of the room, nor did Anderson, pattering after her with a sort of lost puppy-dog look. Shaking his head, John laid the merman out on the floor and drew a folding knife from his pocket. He could only hope that the creature wouldn't panic when he started cutting through the net and injure one or both of them. Setting to work, John found that the nylon cut readily enough. Amidst the gentle snap of breaking plastic, he came to the realization that the merman was not unconscious, as he had supposed, but was watching his every move through narrowed eyes. The expression was not so much one of animosity as it was of wary yet intense curiosity. It occurred to John to wonder if it could understand English.

Finishing his work, he put the knife away and went to transfer the siren from floor to tank when it spoke.

"Don't put me in there." It's voice was waspish, and the human annoyance in his speech made John start. "That's freshwater," the merman continued. "Your friends upstairs just pulled me out of the ocean. Put two and two together."

The doctor blinked dazedly for a moment before replying with, "They're not my friends."

The merman gave what could only be described as a long-suffering sigh. "Ship's doctor?" he asked. "Naturally," he added when John nodded mutely. "Then you understand the concept of osmosis and salt equilibrium. If you put me in there, all the seawater in my cells will leach out and I'll shrivel up and die."

"Right," John said, still processing the fact that the creature not only spoke fluent English but appeared to have likewise a strong grasp of biological sciences. "Right. I'll... see what I can do. Do you mind...?"

He was sure that the merman rolled his eyes. "Yes, actually, I do mind waiting on the floor, but since I don't seem to have a lot of other options at the moment, I'll just have to make do."

John muttered something that might have been "Right," again, before he backed out of the cargo hold and flagged a passing Anderson.

"Oi," he said. "He needs saltwater."

The skipper stared at him. "Why do I care?"

John considered this briefly. Why would Anderson care if the siren didn't survive the trip? "Donovan mentioned a reward," he said. "I bet that the reward is a lot more for a live merman, wouldn't you say?"

The doctor didn't know if it was the mention of money or of Donovan that did it, but Anderson sighed and said, "Alright, I'll draw up a barrel of bloody saltwater, but you get to empty the tank, not me, got it?"

John just nodded and smiled, and watched Anderson stalk off in aggravation. Then he stepped back into the hold and found that the siren had rolled onto his stomach and now was leaning on his elbows, still observing John intently.

The doctor crossed to the tank and grabbed a wash bucket from off a crate. He was just beginning to scoop out the freshwater and return it to its barrel when the creature spoke again.

"I wasn't sure if you would come back." Something about the siren's baritone sent a tingle down the doctor's spine, and he paused for a moment before continuing his task. Water sloshed in the bottom of the barrel. Just when he began to think the merman had said all he intended to, he continued. "I thought you might get distracted. It's obvious that you don't mean me any harm - you're a doctor, for starters, and you didn't try anything with the knife, nor have you made any sort of threat - but I also can't be sure how vested you are in my survival. As a crew member on a separate payroll, it's unclear how much you're going to get out of handing me over to your government for disposal."

John had managed to stay mostly unaffected during the examination of his moral character, but this last comment made him blanch.

"Disposal?" the doctor asked, glancing sidelong at the blue-grey eyes watching him thoughtfully.

The merman snorted. "'A Queen's reward'?" he said. "What did you think she meant, that I was going to be sold to the London aquarium or something? Sirens," he said, with an expression the doctor could not place, "are too dangerous to let live."

"They're going to kill you?" John emptied the bucket into the barrel again before turning it upside down and sitting on it. "They can't - I won't let them."

The merman rolled onto his back and stared at John upside down. "Why not?"

"Because you don't deserve it!" the doctor exclaimed indignantly.

"Are you sure about that?" The siren's voice was soft, but it still chilled John's blood to hear it.

"Were you going to sink this ship?" John asked.

"No," came the whispered reply.

"Then you don't deserve it," replied the doctor firmly.

"And if I had been going to?"

John stood and turned around on the pretense of picking up the bucket, though really he wanted a moment to think. Still, it occurred to him that he probably wasn't fooling the siren, who appeared to be inhumanly clever.

"Even had you been going to..." the blonde man said slowly, "I still don't think it would be right to hurt you for it."

John bent and drew the remaining water from the tank. Perhaps the merman had been intending to reply, but then he suddenly went limp on the floor. For a moment, the doctor was worried - had he been out of water for too long? - but then he noticed the faint smirk on the creature's face and heard footsteps on the stairs. A second thereafter, the door opened and Anderson stumbled through, lugging an enormous barrel of saltwater.

"There," he spat. "And this better be worth it," he said, sparing a glance for the siren. "Stupid git," he added, aiming a kick at the merman's side. The creature's feigned unconsciousness never wavered, but John saw his jaw clench in pain.

"Oi! Lay off!" John said angrily. "He didn't do anything to you."

Anderson shrugged and walked out, but just before he closed the door he turned and said, "Yet."

As soon as the doorknob latched, the siren exhaled and gave a small groan, rolling back onto his stomach.

"Ouch," he said dryly.

"Did he hurt you?" John asked, kneeling.

The merman shrugged. "Not badly."

"I'm sorry," the doctor said, and he meant it. "If I would have known, I would have stopped him."

"It's just as well that you didn't," came the reply. John tipped the saltwater into the tank and with a heave lifted the siren from the floor. It was different carrying him when the doctor knew he was awake. Having those grey eyes look at him made him wonder if the creature was staring through to his soul, and he realized that the merman's eyes were precisely the same color as his tail. There was a dropping sensation in the pit of his stomach that John couldn't place the origin of.

"Why's that?" the doctor asked.

"Because if he knows you sympathize with me, then they'll be even more likely to suspect you after you help me escape."

John released the siren into the tank; the creature settled back to lean against the shorter glass side like he was in a bath tub.

"Escape, huh?" John couldn't deny that he would get the siren out if he could, although it would be nothing but difficult trying to smuggle him past the crew. "Well, if we're to be partners in crime -"

"Or in preventing it," the siren smiled.

"- Then you should know my name," the doctor finished. "I'm John. John Watson."

The siren's smile stretched wider. John, who had expected pointed teeth, was surprised to see that the merman had a completely human set of molars.

"I'm Sherlock."

John nudged the door open to find Sherlock staring blankly at the ceiling. Crossing the cargo hold, he took the bucket for his seat again and sat next to the tank, withdrawing a sandwich from under his jacket.

"Ham and cheese," he said sheepishly. "I haven't had lunch yet. Are you hungry?"

Sherlock's eyes never left the ceiling. "Not especially."

"Come on," the doctor cajoled. "You've got to keep your strength up." John ripped a piece of bread from his sandwich and held it out. The siren's head turned sharply to glare at the offending piece of food.

"What," he asked, "is that?"

"It's bread," John said encouragingly. "Go on. Try it."

"I eat fish, oysters, shellfish, and if I'm truly desperate, kelp," the merman said stiffly. "I do not eat bread."

"Fish love it," John promised.

"I am not a fish." In that moment, Sherlock's voice was as petulant as a displeased toddler's, but he took the offering anyway, and, with an expression of great distaste, put it in his mouth.

"Well?" John prompted.

"Next time, bring some fish," Sherlock told him, and the doctor's face fell. Expression softening somewhat, the siren sighed and added, "but maybe you can bring some bread and show me how to make a fish ham-and-cheese."

"'Sandwich'," corrected the blonde man.

"Sandwich," said Sherlock. "Yes. One of those."

John did, in fact, return with a plate of seafood for the vessel's captive. Sherlock seemed genuinely surprised by the gesture and unsure of how to say his thanks. As he nibbled alternately on a mackerel fillet and a piece of soda bread, John explained.

"The cook, Molly Hooper, saw you while you were on the deck. She's rather enamored, I think, and was only too happy to help me get you some lunch."

"That was... unnecessary of her," said the siren, downing a shrimp. "Won't somebody notice?"

John shrugged. "She's the cook, isn't she?" he asked. "Who else is there to notice?"

As Sherlock ate his way through a plate of raw seafood, a singular thought occurred to John, one which he tried to ignore and found he could not.

Eventually, he asked, "Er, Sherlock... When you guys wreck ships... You don't ever, you know..."

"Feast on human flesh?" Sherlock finished absently, swallowing a sardine and finding that he rather liked them packed in salt. John colored, but nodded. "Mmm, not usually," Sherlock replied, finding another sardine. "Only the really crazy ones do that."

"Right, got it." Another horrifying thought occurred to the doctor, and he blurted, "How did you know what I was going to ask? You're not psychic, are you?"

Sherlock's expression said quite plainly what he thought of that idea. "Not psychic, no." The condescension in the remark stung. "Just observant. Don't blame me for your being so easy to read." There was a silence which bordered on oppressive as John struggled not to be hurt and Sherlock looked progressively more uncomfortable. "Don't be like that," he said finally, his tail shifting so the water around it sloshed. "Nearly everyone is. Besides, there's something I really ought to tell you."

"What?" It came out a little more curtly than John had intended.

"I don't wreck ships." Sherlock said this with the sort of tone humans reserved for explaining that they have AIDS or another equally unpleasant disease.

John frowned slightly. "But I thought -"

"Other sirens do," Sherlock snapped. "But I won't. Can't. Whichever."

John felt a knot of tension he hadn't known he'd been carrying dissipate. "Then you really weren't trying to sink the ship," he said with something approaching wonder.

Sherlock smirked a little, resting his arms on the side of the tank and leaning his chin on them. "Funny that you should help me when you weren't even quite sure I wasn't trying to kill you."

"I didn't think you were." It was John's only explanation. "What were you doing, then?"

"I like to watch people," the siren replied. "You're interesting."

"Sherlock..." the doctor asked again, "If you don't wreck ships, then what about all that other stuff?"

"What stuff?"

John knew that Sherlock knew what he meant, but he replied anyway.

"You know - the legends about raising storms, collecting treasure, seducing pretty young women..."

"Not just young women," Sherlock said with a sly grin. "Usually in the stories it's handsome sailors."

John felt himself flushing heavily, and it wasn't until Sherlock lost his composure and positively cackled with laughter that John realized he'd been had. Suppressing the disappointed flutter in his stomach, John smiled wryly, saying, "You really had me going for a moment there."

Sherlock finished his fit of snickers and said, "I know." Sobering somewhat, he added, "But regarding your question, no, I don't do any of that, either. I'm not a proper siren at all; Mummy is ever so disappointed by it. That's why Mycroft is her favorite."

"Mycroft?"

Sherlock rolled onto his back and struck the water with the flat of his tail fin moodily. "My older brother. He's been the pride and joy of the family since I was still one of the fry. He's got at least six major wrecks to his name, and enough power and influence to make anyone sick."

John was fascinated by the revelation that not only did Sherlock have a family, but that its complexities rivaled those of human social groups. He wanted to ask more, but got the impression that Sherlock was not eager to talk about it.

Indeed, at that moment, the siren turned back to him and said, "But I suppose you know all about family difficulties, seeing as you don't get on with your sister."

John blinked. "Beg pardon?"

"Your sister," Sherlock repeated.

"Harry - Harriet," John explained. "But how on earth did you know I have a sister?"

The siren pointed a webbed finger at John's watch.

"You didn't buy that," he said. "You're a practical man who works on fishing trawlers to earn a living; you wouldn't waste money on accessories when you've got a phone and presumably a computer, both of which tell the time. Ergo, someone bought that for you. Who? Your mother? Unlikely. Mothers go for sentimentality. She would have gotten your name engraved on it. A brother wouldn't have bought something that thoughtful. My conclusion - a correct one, it seems - is that you have a sister."

"But -" John tried to ignore the fact that he was staring at Sherlock open mouthed. "But - how did you know we don't get on?"

Sherlock smirked. "That's easy. The watch isn't set."

John blinked and looked down at the clock face fixed on the metal links. The battery must have run out, he realized, and he had never noticed.

"That..." he said slowly, and Sherlock's face went from triumph to stoic neutrality in a heartbeat. "That was amazing."

For a moment, the siren actually looked taken aback. "Oh. Well."

"What?" John half-laughed.

"Nothing." Then, "You really think so?"

"Of course!" John said, sincerity lighting up his eyes. "It was totally brilliant!"

The tiny smile on Sherlock's lips was better than a thousand "thank you"s.

Sherlock wanted to see John's laptop, but an accident in the freezer room had kept the doctor busy all afternoon tending to a particularly complaint-inclined engineer who had burned his hand when a pipe broke. Moreover, Sherlock had pointed out earlier that it was unwise for them to spend too much time together, lest the ship's officers get suspicious.

Therefore, it was well after curfew when John stole out of his cabin and down the main corridor, laptop tucked under his arm. Across the hall, a light was on and shining through the crack under the Captain's cabin door. The doctor might have passed it by, had not he heard the low sound of arguing voices from the other side.

"- just not sure that it's right." That was Lestrade. Sucking in a breath, John crept cautiously across the floor and crouched so as to hear better.

"You know what they're like." The second voice was Donovan's. "Freaks, all of them. Blood thirsty, ship wrecking -"

"But that's just it!" the Captain interrupted. "Since we picked him up, nothing's happened. I was expecting rough seas at least, or some kind of massive storm, but there's been nothing. Look out the window - the weather hasn't changed since this morning! You're sure it's a siren?"

Donovan sighed. "They're the only ones with that sort of greyish coloring."

"But then why are we still alive?"

"I don't know, okay? He showed up on the radar this afternoon - the fish tracker about had a field day - and then when I saw what he was, we ripped him out of the water. But he's dangerous, Greg. And when we pull in to port we're handing him over to the authorities. I already made the call."

There was the sound of a chair creaking as Lestrade sat down. "But what if he's alright? That's all I can think and it's driving me stir crazy."

"So what do you want to do?" Donovan asked with disbelief. "Throw him back? Even if we made it out alive, you'd have a ship full of people wondering what happened to their holiday bonus. The government pays twice our salary for a live siren."

"You're right, of course. You're absolutely..."

The Captain's voice faded as John hastened down the corridor, panic etched in the lines of his face. He fumbled with the knob of the cargo hold, pushed open the door as silently as he could, and inched his way inside, latching the bolt behind him with a small sigh.

"Sherlock?" he whispered.

There was a splash, and luminous grey eyes blinked at him from across the room.

"John."

"Sherlock, we have you get you out of here," John said urgently. "Tonight, if possible. The First Mate's already called somebody about you."

"I assumed that's why you were rushing in here in such a tizzy." The siren's voice was calm. "Don't give yourself high blood pressure. They're hardly coming to drag me off this minute."

"Aren't you concerned?" John drew his bucket over to where the merman was reclining in the tank. The plastic was developing a permanent dip in it from being sat on.

"Not unduly." Sherlock flashed him a small smirk.

"Well, I'm glad one of us can be so relaxed," the doctor grumbled. "I brought my laptop; you said you wanted to see it."

"Ah. Yes." Sherlock turned on his side, drawing his tail up underneath him. "Let me see, then."

John flipped open the computer, illuminating them both suddenly in the blue glow.

"So. I type in my password -"

"Using those keys?"

"Yes, using the keys. And then I -"

"Is your password, 'Afghanistan'?"

"How did you - Oh, for the love of - Do you want to see how this works or not?"

Sherlock nodded silently.

"Alright." John opened Safari. "You can look up whatever you want in the search bar, and it'll find all sorts of information about it."

"Let me see." The siren wrenched the slender device from John's hands, balancing it on the edge of the tank. "Now I can -"

"Hang on," John interrupted. "Remember what I told you? Water and electricity don't mix." He prized Sherlock's fingers from the keyboard, snatching the laptop away before the siren could drip on it. "Your hands are cold," John remarked, his digits lingering on Sherlock's a moment longer than a human would have deemed proprietary, but the siren didn't seem to mind. Indeed, as John made to pull away, Sherlock grabbed him by the wrists.

"Cold-blooded," he said. "Fish, remember? You're warm."

John shifted awkwardly in his seat, feeling the onset of another flush and glad the light was largely nonexistent. "Yeah, well, mammal," the doctor replied. "Warm-blooded, remember?"

Without really thinking about what he was doing, John ran his thumb over the webbing between the fingers gripping his opposite hand. It was only when Sherlock shivered that John startled and stopped guiltily. The merman, however, shook his head slightly. "The skin there is sensitive. I'd never noticed before."

"Then... it's alright?"

Sherlock cocked his head to one side. "You're warm. It's alright."

John returned with a not-small degree of trepidation to rubbing gentle circles on the alien flap of skin. He had expected it to feel slimy, given its color, but it wasn't - just thin and cool and translucent, threaded with tiny veins. If he hadn't known better, he would have thought the siren gave a small sigh of contentment. Even if he imagined it, it nevertheless gave the doctor the encouragement he needed to brush a finger over the small opalescent scale set in Sherlock's knuckle. This time, he was certain the siren exhaled more heavily, and, clasped wrist to wrist, John noted that his wasn't the only pulse which seemed elevated.

Feeling ever so slightly more daring, the blonde man took a breath and ran his forefinger up across the top of Sherlock's hand. His touch was feather-light, tracing the pattern of subcutaneous blue veins. John glanced up; Sherlock was staring at him, the full intensity of his penetrating gaze concentrated on the doctor, but the siren made no move to stop him. Looking down again before he lost his nerve, John let his hand continue upwards, tracing the path of the tiny scales which glittered in the artificial light. As John's fingers came to the crook of Sherlock's elbow, the siren's hand moved convulsively, grabbing the straying appendage once again around the wrist. For a moment, John was terrified that he had overstepped some invisible boundary, a boundary he was only just realizing how much he wanted to test, when Sherlock drew the doctor's arm toward him and pressed John's hand to his neck. The siren's eyes fell closed, and a soundless gasp escaped his lips.

"So... warm..." he murmured. "So..."

Sherlock's eyes opened slowly, making John acutely aware of the foot of space between their faces.

"Um," John stuttered. "That is, can we - can I -"

"John?"

"Mmm?"

"Shut up."

The one hand of Sherlock's which had been holding the doctor's left arm prisoner all this time released its grip with a suddenness that was alarming, attaching itself instead to John's neck, pulling him closer, and then Sherlock's lips were on his. Somewhere in John's brain, a neuron fired and he realized what just happened. Instinct kicked in where shock was still filling out the mental processing forms, and the doctor slid his hand up Sherlock's neck, wrapping his fingers in a handful of jet curls. The siren slid his tongue between John's parted lips, and the blonde man tried to pretend that the ensuing whimper had not come from his own mouth. Then John felt Sherlock's arms drop to his sides, and a moment later he was being dragged into the tank.

With a muffled "mmmf!" and a heavy splash, the doctor landed on top of the siren, legs inadvertently straddling Sherlock's powerful tail. John scarcely had the chance to notice before Sherlock was occupying all of his attention again, pulling the blonde man's (now wet) jumper over his head like he had a personal vendetta against it. A mere moment after the siren discarded the forlorn garment on the floor, John was shivering.

"Water's c-cold," the doctor said, wrapping himself more securely around the merman.

Sherlock gave a throaty chuckle. "Warm it up, then, you dolt."

"Challenge accepted," John replied breathlessly. He leaned forward and nipped at the siren's collarbone, relishing Sherlock's surprised exclamation and the way his tail thrashed in response. He bit harder, raising a pink mark on the merman's skin. Sherlock actually groaned at that, and suddenly the water wasn't cold, but boiling hot, John was sure.

One of the siren's hands was in the doctor's hair, the other on his waist, as John's teeth left a trail of pink spots all down Sherlock's ribcage. Sherlock grabbed John hard by the shoulders, pulling him closer, and then with an undulation of his tail flipped the pair over so that John was the one pinned against the wall of the tank, trapped beneath coils of smooth scales and hard muscle. The merman kissed John again, bracing himself against the glass; the doctor bit Sherlock's lower lip and was rewarded with a small, needy sound.

Shifting over to suck on the inside of the doctor's neck, the siren pushed John a few inches deeper into the water. It was around his shoulders now, and it occurred to him as he gasped with Sherlock's touch that the raven haired creature had him exactly where he wanted him. All Sherlock had to do was get John to slide down a few inches more, and the doctor would most assuredly drown, trapped in a foot of water underneath the weight of the legend caressing him. The blonde man could not find it within himself to be properly frightened; for the first time since the war, he felt alive again.

Under the water, Sherlock had found the zipper of John's trousers and was trying ardently to figure out how it worked; squirming as the siren's insistence brushed electrically against charged nerve endings, John worked his pants off himself, canting his hips towards the sinuous body above him. With an expression of complete and total ecstasy, Sherlock took John in his hand, and John drew blood biting his lip to stop himself waking the ship with a cry. Hands gripped scales, webbed fingers worked their task in the gloom, and as John collapsed, the white fire of climax rolling through him, he felt more at peace than he had in years.

Sherlock curled around him, nuzzling his face against John's chest, muttering "mine-mine-mine" in a breathy mantra.

"Yours," John agreed softly. "But you're mine, too."

The siren seemed to consider this for a moment.

"John's," he said, rolling the name off his tongue. "Mmm. John's."

"And you," the doctor said, running his fingers through Sherlock's hair, "have to get out of here. It isn't safe."

"Is this the part where we say our goodbyes and part ways?" asked the merman, wrapping his arms tighter around John's chest.

The blonde man found he had scarcely thought about it, but knew immediately what he wanted the answer to be. "Would you like to?"

"Don't leave me," Sherlock practically begged.

"I won't," John promised.

"I'll... live in your shower or something," the siren said, although his tone of voice bespoke his distaste for the idea.

John laughed. "You would be bored out of your mind," he said. "Maybe I'll rent a place with some private seashore."

Sherlock hummed in agreement, his breath soft and cool over the doctor's wet chest.

There was a kink in John's neck, but he was relishing the sensation of being totally entwined with the merman too much to bother moving. Scales slid over skin as Sherlock shifted his weight, and in the cooling water, the feeling of skin-on-skin was the single most pleasant thing John could remember ever experiencing.

"I think you're a bloody good siren," said the blonde man, still sleepily content in a post-coital daze.

"Oh?" Sherlock looked to be in much the same state as John, and his eyelashes tickled as they brushed against the doctor's side.

"Mmm. You're awfully good at the 'seducing handsome sailors' bit."

The merman tisked. "Hardly. You're still breathing, for starters."

The doctor stretched. "That's usually a good thing, at least where human partners are concerned."

Sherlock raised his eyebrows. "Hmm. And here I'd always heard that relationships were suffocating."

John snorted, and before long they were both giggling helplessly in the water. They were only interrupted by voices outside the door.

"- was that?" It was Donovan.

John and Sherlock stared at each other in mutual horror, each realizing simultaneously that they hadn't exactly been being quiet.

"Not sure," came Lestrade's muffled reply. "Sounds like it came from in here."

Sherlock reached outside the tank and slammed the lid of the laptop down, then dragged John to his other side, where he was hidden behind the merman's bulk. As the doorknob turned in the darkness, all the deeper for the absence of the computer's glow, the siren positioned himself in the doctor's lap so that John was all but crushed beneath him, scarcely able to breathe, but concealed from sight.

The door swung open, and the gaslights in the hall illuminated the respective profiles of the Captain and First Mate as they stared into the small hold.

"What're you doing in there?" Lestrade asked suspiciously. Sherlock did not answer, but cocked his head as if he did not understand. The Captain rolled his eyes in exasperation, turning to leave, but Donovan was not so easily dissuaded.

"It's up to something," she said. "Is it safe to leave it untethered?"

"Dr. Watson seemed to think it safe enough," Lestrade replied doubtfully.

"Yeah, well, he would, wouldn't he? Primum non nocere and all that?"

Donovan stepped further into the room, and John felt the muscles in Sherlock's back tense. It was not lost on either of them that the doctor was not especially well-hidden; all the First Mate had to do was look over the siren's shoulder and he'd be caught. Moreover, the other evidences of John's presence were not very inconspicuous. His jumper still lay abandoned in a heap on the floor, for heaven's sake.

Sherlock whipped his tail against the glass, sending water splashing in Donovan's direction.

"Sally!" hissed the Captain.

With an expression of deep disgust, the woman stepped back, wrinkling her nose. "Ungrateful creature," she spat. "It'd be dead already if the PM would still pay as well for the corpse."

She pulled the door shut so hard behind her that it shook the frame. John stared after her in shock.

"You've got to go, Sherlock," he said. "Now. She's out of her bloody mind."

The reply was brisk. "Agreed. Pull your pants up, but leave the jumper - it's soggy and will just make you colder."

"Right." John eased his way out from under Sherlock, fixing his trousers as he stepped out of the tank. "Should be simple enough," he commented. "I'll just dump you over the side, shall I, and then go to bed. No one will be the wiser."

"And then tomorrow..." If John didn't know better, he would have thought that Sherlock's too-casual tone was masking some lingering uncertainty, but it was difficult to envision the vain creature as being anything but confident.

"Tomorrow?" John prompted.

The siren tossed his curls from his face, shuffling slightly. "Well, you're returning to London, I suppose?"

And suddenly, John grasped the implication behind Sherlock's veiled questions. "I'll meet you," he promised. "Down the coast, to the south of the port, there's some private property with beachfront access. No one will know if I meet you there."

Sherlock gave a crooked smile before looping his arms around John's shoulders.

"It's a deal," he said. "Now watch the door so it doesn't creak when you open it."

The passage through the hall was as fraught with anticipation as any John had ever experienced. At any moment, he expected to be set upon by one of the crew. The lights were dim in the hall, and every shadow could have masked a dozen men. Yet in spite of this, they met no one, and with a quick jaunt up the metal stairs, they reached the deck.

The moon was nearing fullness and hung over the open ocean, the black sky glittering with the whole of the Milky Way, and the light spilled across the deck. It glistened on the metal, slick as it was with spray, and left few shadows in which to crouch. The light was still on in the bridge, but the room was empty, so it mattered little.

Sherlock's ichthian weight was heavy in John's arms as he set him down on the side of the trawler, leaning him back against the short rail.

"You won't forget?" the merman asked, too visibly nervous to be coy, and the doctor wondered how someone so brilliant could be simultaneously so insecure.

"I couldn't if I tried," John replied emphatically. "You'll be safe?"

The siren's small, off-kilter grin was back, and John fought off the urge to snog him again.

"Thanks to you," Sherlock answered. "So I'll see you -"

He broke off, his face turning ashen in the moonlight as his face tilted up toward the bridge. John whipped around, and locked eyes with Sally Donovan, who stood framed in the window and even at that moment was relaying something over a radio, doubtless informing Lestrade that the doctor was helping their prisoner escape.

"Go! Quick!" John hissed, turning back to Sherlock. "They'll be here any minute!"

"They will," Sherlock nodded gravely, "and then you'll be the one in danger. I don't know what the Brits' penalty for freeing a siren from captivity is, but it amounts at the very least to aiding and abetting murder."

"I don't care," John said, clenching his teeth.

Behind him, the doctor could hear feet on the stairs.

"Do you trust me?" Sherlock asked, his voice low. The man nodded tightly. "Then relax," whispered Sherlock. "Wrap your arm around my middle and stare straight ahead."

John almost questioned him. Instead, he slid his arm around the merman's narrow waist and stood stock still as Lestrade arrived, panting, on deck. The Captain stared in shock at the pair of them.

"Dr. Watson!" he called, stepping closer. "John! Come away from there - that thing could kill you."

I know, thought the blonde man. But I don't care about that, either.

"John?" Lestrade looked genuinely concerned as he took another step towards them, stopping dead in his tracks when Sherlock let out a shockingly inhuman growl.

"Too late," the siren half-sang, and a chill ran down the blonde man's back. "He'ssss mine." If Sherlock was acting, he was brilliant. Had John not spent all day in his company, he would have believed in that moment that the siren meant him incontrovertible harm. As it was, the adrenaline rushing like a drug in his veins was a siren song in its own right.

Lestrade was saying something ("Let him go, you bastard!") but John hardly noticed, too enraptured by the splay of Sherlock's fingers over his ribs.

Then Sherlock, who had all this time been inching slowly closer to John, pulled the doctor close and gave a powerful push off the wall with his tail, sending both man and siren backwards over the side. John and Sherlock fell down past the sloping sides of the trawler, sinking into the black water even as Lestrade rushed to the edge after them. Staring down at the ripples in the Atlantic, the Captain hung his head.

"Poor bugger," he muttered.

Under the water, Sherlock's lips were pressed to John's as the one tried to breathe oxygen into the other. Eyes drifting absently over the siren's shoulder, John watched the ship's keel grow smaller above him as the light faded around him and pressure built in his temples. For all Sherlock's ministrations, they were sinking too far, too fast. The cold stole into John's bones, and his chest felt tight.

He was drowning, and he knew it. Still, in the split seconds of consciousness remaining to him, he knew he would rather die then and there than be back on the trawler, never having met Sherlock. He tilted his head, turning the merman's life-saving measure into a proper kiss. Water flooded his mouth, and he choked. As his vision turned hazy, the water changed around him, but he hadn't any time to contemplate it as he lost his grip on reality and fell again, this time into unadulterated blackness.

There was sand in his hair, and the sound of waves in his ears. John blinked, squinting against the sudden glare of sunrise. A figure was bent over him, brushing grit from his face, and as John's focus returned to him, he decided he knew exactly who it was, if the heaviness draped over his waist were anything to judge by. Water rushed in with the tide, brushing tendrils of foam over the doctor's knees, and he blinked again.

Sherlock was practically lounging across John's lap, and if it wasn't for the hazy concern written in his piercing grey eyes, the blonde man might have found it easier to imagine himself a siren's conquest rather than friend.

"Where -" John croaked, and cut himself off with a grimace. His mouth tasted like salt and his tongue was as good as lead in his mouth. "Where am I?" he tried again.

Sherlock shrugged. "The private beach was your idea. I didn't think anyone would be likely to find you, though - no signs of life more recent than two weeks old - so I stuck around to keep an eye out."

John sat up, taking in his surroundings: a long stretch of beach curved away beyond a bluff, upon which sat a sprawling white house. Beach grass softened the hillside, and it was utterly abandoned. Nevertheless, Sherlock had dragged John up alongside a small wooden pier, the sort one would anchor a single man boat at, in the hopes of providing them some cover.

"Am I dead?" the doctor asked, frowning down at his unmarked flesh.

Sherlock smirked. "I hope not. I rather disapprove of necrophilia."

"Why am I not dead? I mean," John hastened to clarify, "I'm perfectly glad not to be, but I was drowning."

The siren coughed slightly. "My brother may have had a hand in that. Doubtless my disappearance put him in a frenzy and then he felt somehow indebted to you for saving my life, so he returned the favor."

"Marvelous," John groaned, flopping back into the warm sand. "I've been nearly killed and supernaturally resurrected and it isn't even... 7:00 yet."

Sherlock planted his hands in the sand on either side of John's face and leaned forward.

"There's worse ways to spend a morning," he commented.

John hummed his concurrence, wrapping his fingers in Sherlock's hair.

"There are at that," said the doctor. "Now shut up and kiss me, you brilliant idiot."

Sherlock proceeded to do just that.