The doorbell for 221B rang yet again. Sherlock tried to ignore it, as he had ignored it the last several dozen times. John looked up from his laptop.

"It's your turn, Sherlock. The candy bowl is down by the door. I did it for an hour already earlier."

"I'm not taking a turn," Sherlock replied grimly. "It will only encourage the little monsters." His statement was punctuated by another ring.

"Where the devil is Mrs. Hudson?" Sherlock snarled. "Shouldn't she be handling this?"

"She's at a party. Are you saying that you can't handle small children dressed in sheets, Sherlock?" John said, amused. "Maybe you can get yours on and go out yourself." Sherlock winced. That afternoon at the palace had not exactly been his most dignified moment.

John went on, speaking over the insistent sound of the bell yet again. "Come on, Sherlock, it's just a harmless tradition. But if you don't want them to keep ringing, we need to turn out the lights and just go to bed early."

"I'm not going to do anything differently than I would any other night," Sherlock insisted. The bell rang again, held for at least three seconds.

"Wouldn't you answer the door on any other night?" John said, reasonably. Sherlock glared at him, and then went down the stairs. He'd show the brat on his doorstep what a Halloween fright could be like. He flung open the door, prepared to let loose with a stream of invective.

At first he didn't see the child, but then he looked down. Looking up at him was … a miniature version of himself. He widened his eyes in surprise. There was the dark curly hair, and something that looked like his favorite blue cashmere scarf. Astonishingly, the child wore a small version of his own coat, and a neat dress shirt the exact shade of his favorite purple one – the one he wore at the moment, in fact – could be seen under it. The child's serious expression looked hauntingly familiar; he'd seen it recently in his own bedroom mirror. And, most disconcertingly of all, the kid stood calmly, with hands clasped behind the back, as he often did.

"Who … are … you?" he managed.

The child pressed both palms together, placing fingertips under the chin, and said, thoughtfully, "I should have thought that would be obvious. What must it be like in your dull little mind?"

Sherlock took a step back, nonplussed. He wasn't certain how to handle this. "John," he called up the stairs uncertainly. "John, could you come down here a moment?"

"What is it, Sherlock?" John called back.

"Just get down here," Sherlock snapped. He kept his eyes on the child, who had resumed the hands-behind-the-back posture and was regarding him critically.

John came down the stairs. When he saw the child, he stopped three steps from the bottom. He looked surprised for a few moments.

Then, completely ignoring Sherlock, he came down the last steps, approached the child and spoke directly to the little detective. "Yes, Sherlock, what did you need me for?" Behind him, Sherlock's eyebrows rose at John questioningly.

The child smiled. "John, thank you for coming. I have need of your assistance. Could be dangerous, of course," the mini-detective added casually.

"Not a problem, you know that," the doctor replied amiably. "Where are we going?"

"New Scotland Yard, of course," the shorter Sherlock replied. "I'd like to have a word with Inspector Lestrade." The taller Sherlock had recovered his composure enough to roll his eyes, but John continued to ignore his friend. At a movement from across the street and out of Sherlock's line of sight, he looked up to see a woman with a face very similar to the child's. The child's closely supervising mother, John saw. She waved apologetically and shrugged. Clearly, the child had insisted on playing a game, and the mother hoped they'd go along with it, at least for a while. John, seeing her, smiled to himself. Maybe he'd have some fun. After all, it was a night for tricks.

"By all means, then, Sherlock," he said. "Lead on." The child set out, accompanied by John. Sherlock hesitated, and then followed, closing the door to the flat behind them. The mother shadowed them, but kept to the other side of the street and well behind them, always with other people between her and the little group.

As they walked, John said conversationally, "Correct me if I'm wrong, Sherlock, but I believe you also have another name?" The child looked sideways at him, and said confidentially so that only he could hear, "Lydia. But tonight, it's Sherlock."

"Quite an understandable choice," John agreed. "What were the other options for tonight's name?"

"Only this one for several months now," the little girl said, continuing her remarkable imitation of Sherlock's mannerisms. "But when I was younger I wanted to be a pirate." Behind her, Sherlock coughed and stumbled a bit as he heard this. Looking up at John, his small twin smiled a small, self-satisfied half-smile. John nearly laughed out loud at the spot-on resemblance, but controlled himself in time.

"And why are we headed to the Yard, exactly?" John asked.

"Why John, I would have thought that was obvious," the young detective replied in surprise. "The staff there is handing out exceptionally generous handfuls of candy this evening."

"They are?" John asked in genuine surprise. "How did you guess that?"

"I never guess," she replied archly. "I deduced it." From behind them, Sherlock gave a derisive snorting sound of disbelief.

John turned and gave him a warning look, and then addressed his young companion again. "I'm sorry, Sherlock, I wasn't able to follow your line of reasoning. Could you fill me in, please?"

The mini-Sherlock looked up at him gravely. "John, I'm disappointed in you. You see, but you do not observe."

"What should I be observing?" John asked, curiously, smiling inwardly at her remarkable ability to act exactly like his friend. He heard Sherlock draw closer by a couple of steps; he wanted to hear the answer, too.

"Why, John, merely the sizes of the bags carried by the northbound-walking costumed children as compared with those of the southbound ones." John didn't understand, but he also realized that he didn't hear Sherlock's step still behind them. Turning, he saw the real Sherlock had halted, astonished. Then, with quick strides of his long legs, he caught them up. Leaning to whisper in John's ear as they walked, Sherlock said, "She's right, John. As we approach the Yard, the percentage of children with heavily-laden bags increases dramatically."

In due time, they arrived at New Scotland Yard. During the walk, John had taken a few moments to conduct a brief text exchange.

Hi Greg,

Are you busy? I'd like to bring a visitor.

John

No, come on up. Who is it?

GL

You'll see. Play along.

John

The three of them, followed inconspicuously by Lydia's mother, went up to Lestrade's office. Upon their entrance, Lestrade stood up from behind his desk, looking startled to see the little consulting detective. But then, seeing John and the more-familiar Sherlock behind her, he recovered.

"Why, hello, Sherlock," he greeted the smaller version. Like John, he now pretended the larger one wasn't even there. "To what do I owe the pleasure? Not a murder I've not heard about yet, I hope."

"Nothing so dramatic," the little one replied solemnly. But before she could say anything else, Sgt. Sally Donovan ran in, pushing the others urgently aside in her haste to get to Lestrade.

"Excuse me, sir, but we have a crisis on our hands - a bomb threat. This was given to the manager at one of the Sainsbury's." She shoved a piece of paper in front of Lestrade. Instantly, Sherlock stepped forward, and they looked at it together.

"It's from the Music Bomber," Lestrade said.

"Obviously," Sherlock murmured. "A clue as to where to find the bomb, as in each case before." This was printed on the note:

If you try to alert the shoppers or to evacuate the store, I'll set off the bomb. If you don't find it and disable it by eight o'clock, its timer will set it off. Maybe Sherlock Holmes can help you.

After this, there was a musical staff, studded with seven notes.

"It's fortunate you're here," Lestrade said. "There must be something about this clue that's directed specifically at you." His expression was carefully neutral. Sherlock had been able to figure out two of the last three of the Music Bomber's notes in time, but not the last one. Dozens of people had died. He hadn't allowed himself to show it in front of Anderson and Donovan and the other officers, but John had let Lestrade know that later, back at the flat, Sherlock had become quite upset over his failure to save them. And now, the Music Bomber was challenging him by name.

Sherlock narrowed his eyes at the paper. "Nothing leaps to mind about it immediately." Quietly, his little duplicate tiptoed over and unobtrusively peeked at the paper in his hand.

"What about the notes?" Sally demanded. "Don't you play the violin, freak?"

Sherlock ignored her, but the girl spoke up. "Don't call me a freak," she said to Sally with dignity. "And you need to do your research. I'm a high-functioning –"

"Someone get this child away from me," Sherlock growled, stepping away and lifting up his arms as if they might be contaminated by her touch. "Enough is enough. Fun's over. I'm working now, and she's putting me off." His smaller mirror image looked hurt. He turned on Donovan. "Of course I've already considered that," he snapped. "They don't mean anything, and they won't while you insist on distracting me."

John looked apologetically at the girl as he led her back to her mother. "I'm sorry, Lydia – I mean, Sherlock. And you do make a fantastic Sherlock," he smiled. "I wish I had my camera, but more than that, you've got the way he acts down perfectly. But this is serious. You have to go now."

"But, John," she protested, looking back toward Lestrade's office. They'd closed the door, but Holmes, Donovan, and Lestrade could be seen through the glass, clearly arguing.

"Not now, Lydia," her mother said firmly, taking her hand and starting to lead her away. "We'll come back another time."

"But, Mom, they don't realize what the note means," the girl said urgently. "I'm sure I can help. I felt like I almost had it."

"Come on, Lydia. This is Sherlock Holmes, the real one. Do you really suppose he doesn't know his business?" her mother said sharply. "Let's get out of the way before we distract these people. They have a bomb threat to contend with." She pulled on her daughter's hand, but Lydia suddenly yanked free and ran back to Lestrade's office.

It was empty. While she'd been speaking with her mother, Sherlock, John, Sally, and Greg had disappeared to the police car garage, heading toward the threatened store.

Seeing her mother approach angrily, Lydia dashed away again, this time heading for the stairs. Faster and more reckless, she reached the lobby first. Pausing at the drinking fountain, she quickly splashed water on her face in such a way that it appeared that she'd been crying. Breathlessly, ran up to the desk and spoke to the receptionist there, trying to sound distraught.

"My Mommy's at the Sainsbury store on Wilton Road," she said hastily, naming the only Sainsbury supermarket she knew about. "Will she get blown up?"

Obviously word was getting around, the receptionist thought. We're not supposed to mention anything about the bomb threat, but clearly this little girl already knows about it. Well, at least I can put her mind at ease about her mom. "No, sweetheart, the threat is against the one on Victoria Street," the woman said kindly, tilting her head in its direction.

"Thankyouverymuch," Lydia called out over her shoulder. She ran out of the door, just as her mother appeared from the stairwell.

Quickly notifying several police officers about her runaway daughter, Lydia's mother recruited them to assist her. She stopped running after the girl, realizing that, even if she were somehow fast enough to catch up (which she doubted), she didn't want to be chasing her into intersections and risking that she might get hit by a car or by one of London's famous buses. The chagrined receptionist told them where Lydia was likely headed, and the officers got on their radios.

Meanwhile, Lydia was indeed running as fast as she could toward the threatened store. It was about a half-mile away, she recalled from her earlier walk around London with her mother. Fortunately she was in good shape, and she made quite a sight sprinting down the sidewalk, her black coat billowing behind her and the blue scarf flying like a pennant. Each time she came to an intersection, she carefully crossed it; it wouldn't do to get in an accident before she could get to the crime scene.

As she ran, she recalled the bomber's note from the brief glance she'd had. She almost understood it. There was something about it that she could figure out, she knew, if only she had enough time. She was able to picture the little snippet of the tune in her head. It didn't make sense, she thought, mentally playing the notes in her head as she would on her small plastic flute. She didn't recognize the tune, so no lyrics came to mind. Her lean legs carried her ever closer to the store as she pondered what it could mean.

Two blocks away, she reluctantly shed the distinctive coat and scarf, and peeled off the purple shirt. She snatched a ballcap from a sleeping homeless person – "Sorry, it's an emergency, I'll return it later," she whispered – and put it on over her dark hair. The officers would be looking for a child that was dressed like Sherlock, but if they looked in her direction now, all they would see would be another kid in an ordinary T-shirt and hat. She was in a hurry, but she didn't dare draw attention to herself, so she strolled the last bit of the distance to the store.

She looked up at the digital clock on the front of the bank across the street. It said 07:57. Three minutes to go, then. Think, she ordered herself.

She stopped in the middle of the sidewalk and, heedless of being in the way, immediately went to her mind palace. She'd discovered long ago that she had one, just as she'd heard that her hero did. And, it seemed to work just as well. In it, she brought up an image of the note again, and studied it carefully. Suddenly, she knew what it meant.

She opened her eyes, and saw Sherlock Holmes, some distance away. He was standing alone, clearly still trying to figure out the note in the remaining few seconds. He was looking agitated, though, and, behind him, John, Greg, and Sally stood looking panicked. Lydia broke into a run. Perhaps she could reach him in time.

She was still about a hundred feet away when strong arms grabbed her from behind, stopping her. A sharp-eyed London police officer, alerted by radio to look out for her, hadn't been fooled by her change of clothing and he'd caught her.

Lydia tried to scream, but he clamped a hand over her mouth. He leaned down to speak to her quietly. "I'm not going to hurt you," he said kindly. "But you must be silent. Mr. Holmes is trying to figure out how to save all those people in the store, and you mustn't distract him for a second." Lydia struggled desperately. It would hardly be called distracting if she was telling him the answer, she thought wildly. Then, she knew what to do. She'd only have a moment, but it would be enough. She drew in a big breath through her nose, and then bit the officer's hand, as hard as she could.

Startled, he let go with a cry. As soon as his hand was away from her mouth, she shouted at the top of her lungs:

"Sherlock! Viola clef!" And then no more, as the officer's hand slapped down onto her face again.

Lydia anxiously watched Sherlock over the officer's fingers. Everyone else within earshot was looking in her direction, surprised by her outburst, but the detective didn't appear to react. Had he even heard her? Was his attention elsewhere? Then, she saw his chin lift as he took in what she'd shouted and understood it. Lydia nearly fainted with relief.

Suddenly, he was racing full tilt into the store, while the others looked on in astonishment. Through the glass doors, they saw him arc toward the butcher's counter and gather up a large knife without even slowing down. With it, he ran past the checkout stations, knocking people over right and left, and descended upon the produce section. Looking quickly around, he spotted the bin he wanted. Frenzied, he began sorting through it, shoving and tossing things on the floor while the shoppers around him started to object, but seconds later he found what he wanted. He plunged the knife into it, sliced it the rest of the way open, and pressed his thumb against a red button, just as the clock on the bank building clicked over: 8:00. Sherlock stepped back and leaned against the edge of another bin, looking haggard but taking giant breaths of relief after his exertions. Outside, the police broke into spontaneous applause and jogged toward the doors of the store to congratulate him and to assist the formerly doomed shoppers. But John Watson walked toward Lydia, a huge, warm smile on his face.

"A cabbage?" Lestrade said, looking at it in disbelief. The bomb disposal squad had safely removed the explosive device, but the vegetable itself lay cut open on the hood of the police car. He looked up at Sherlock. "How did you know it was hidden in a cabbage, for God's sake?"

He sighed. "I didn't," he admitted. "The other Sherlock did, whoever she is." He indicated Lydia, standing nearby and now wearing her favorite outfit again, as well as a rather familiar demeanor.

"Explain, please," Lestrade said to her respectfully. "That's what consulting detectives do after they save the day."

Lydia turned to Sgt. Donovan. "In the Inspector's office, I started to tell you. I'm not a freak, but I am a high-functioning violator." The sergeant looked confused.

Sherlock looked amused. "Or, to put it another way, a violist - a musician who plays the viola. She shouted 'viola clef' at me, just in time."

"So?" Sally demanded. "What does that have to do with anything?" The two Sherlocks smiled at each other in mutual understanding, but the police officers and John were just as confused as before.

"In that musical clef, the notes spell out the word CABBAGE," Lydia's mother explained into the silence, coming up from behind Sherlock. He whirled. "Grace!" he exclaimed. "I didn't realize you were in London!" The two friends exchanged a brief hug, but then Sherlock blinked in realization. "Oh," he said. "I should have known."

John chuckled and stepped forward to greet Grace as well. "Welcome back to London, Grace," he said warmly. "It's good to see you."

Sherlock looked at Lydia, and spoke in an aside to Grace. "Her wardrobe is … I was going to say unique, but that's exactly the wrong word. Your doing, I surmise."

Graced laughed. "No, actually her father's. He finally lost three chess games in a row to her, and this was what she asked for as a reward for the achievement."

"Children commonly beat their parents at chess," Sherlock remarked.

"Not in our family," Grace said. "He'll be in your section at the FIDE tournament in Paris this weekend. You might get to play against him."

Sherlock looked at Lydia with new eyes. "D4!" she suddenly chirped.

Sherlock looked at her sternly. "Cambridge Springs Variation …"

Lydia looked delighted. "… of the Queen's Gambit Declined! Okay!"

"Mainline variation, move eight, and you play Black," Sherlock challenged her. "Text me your move by midnight." Lydia happily nodded her agreement. "And no help from computers," he added. Lydia rolled her eyes and looked at her mother.

"Lydia has an arrangement with one of the database providers," Grace explained to Sherlock. "They hire her to work out the lines of the various openings. It's the computers that use her data, not the other way around."

There was a sudden flash as a camera snapped a picture. Sherlock blinked. John thanked the police officer, who promised to send an email with an attachment later that evening, and walked off. John chortled. "Oh, I do love that look, although it's pretty rare. Sherlock Holmes, dumbfounded and at a complete loss for words. I'll treasure that picture."

In a stage whisper, Grace leaned over to Lestrade and said, "If any of the boys down at the station start betting on the outcome of this game, put your money on Lydia. That opening is the family specialty."

Sherlock, vexed, looked again at Lydia, who was now looking quite smug. "Why isn't she telling me her next move now, then?" he asked.

"I imagine she intends to go home and look up your tournament game history," Grace told him. "She'll want to study your playing style and develop a counterstrategy."

"I foresee a certain amount of time in study for myself as well, then," Sherlock said, regaining his dignity. He looked steadily at Lydia. "Game on, kid."

Turning back to Grace, he said, "The wardrobe aside, her impression of me is … remarkable. There's only one person who could have taught her that." He looked at her meaningfully.

"At least two, actually." John stepped forward and put an arm around Lydia. "Why, Sherlock," he said, as the detective looked shocked. "Haven't you ever heard of Skype?"

Sherlock looked back and forth between Grace and John. "It's a conspiracy," he groaned.

Grace walked over to her daughter and hugged her proudly. Then she led her back over to the detective. "Sherlock, meet my daughter Lydia. Lydia, this is Mr. Sherlock Holmes." Lydia grinned and held out her small hand. Sherlock took it, and shook it sincerely. "Congratulations. You solved the clue, and took decisive action that saved many people's lives," he said, impressing John by displaying some manners for once. "Perhaps your mother will allow us to work together again sometime." Grace looked doubtful. "Or, perhaps not until you're older. As you noted yourself, sometimes this work can be dangerous."