A/N: This was inspired by a prompt castleholic (aka the amazing 'sugar free vanilla' here on ff net) left on the castlefanficprompts tumblr page about pre-couple Caskett meeting the couple/engaged/married versions of themselves through time travel, but the idea quickly spiraled into something else and it's... different. But I just can't get enough of Captain Beckett and that AU world.

Set anytime after the events of 7x06, The Time Of Our Lives.

"What if we're in an alternative universe?" Castle whispers at her side, glee radiating through his voice and she shakes her head, even as her heart flutters with ridiculous affection for the childish man trotting next to her.

"Castle, we both got knocked on the head. We were out for a while, doesn't mean we've been transported to some parallel universe."

"You would know all about those, though, wouldn't you, Captain?"

She rolls her eyes and quickens her stride down the sidewalk, hating him more each time he brings it up. She never should have confided in him about 'the other Castle', but ever since she had met him (again) and he had wormed his way into her precinct (again), she had found that she trusted him. It wasn't the same, but when Richard Castle looks at her, there's a hint of that breath stealing longing she had seen all those months ago when a different version of the same man had waltzed into her life claiming to be her partner.

It sounds crazy when she thinks about it, she's still not wholly convinced it wasn't all a dream, but this Castle - the one she had met by sheer chance in a coffee shop only a week and a half after his shooting - had eradicated her doubts. The first time she had seen him, sitting alone by a window in the shop, his laptop untouched on the table in front oh him, her heart had stopped because he was alive, but then he had noticed her and intrigue had flared in his eyes, but nothing more. No recognition.

That hadn't stopped him from rising from his table and attempting to flirt with her.

He's been fascinated with her ever since, cashing in on an old favor from the mayor and managing to weasel his way into shadowing her despite how boring her job can be. It was in her office one day, six weeks after the day she saw him for the first time again, that she ended up telling him.

She had expected laughter, ridicule over the story of another him from another world, but he hadn't laughed, hadn't made fun of her. He'd touched her hand and promised that he wasn't going to leave her. Never again.

And that's where they were now, balancing on the edge of more than friends but less than lovers, because they had both agreed to take things slow, to do it right. Castle doesn't mind. After what she's shared with him, he's convinced that she's it for him, his destined other half, and as corny as it sounds, she believes it too, believes that maybe he really is her one and done. Maybe he always has been.

"But things are different here. The precinct was different, that other woman saying she was the captain…" Castle murmurs when the loft comes into view.

She's been to his home plenty of times since they met, sharing dinners with him and his family, sometimes indulging in a movie with him on the couch before she forced herself to get up and go home.

Last time was the first time he had kissed her goodnight.

The memory seems to hit him the same moment it hits her and she blushes when he grazes his knuckles along the back of her hand.

"Mr. Castle?" The doorman greets him with a puzzled look once they reach the building's entrance and Castle guides Beckett inside with a nod to the other man.

"That's not my doorman," Castle hisses as they walk a little too quickly towards the elevator.

"They probably just hired someone new," Beckett reasons and it's true, just because there's a doorman he doesn't recognize manning the entrance doesn't mean they're in a different world.

Castle captures her hand as the lift starts to ascend and she nearly startles, but he only tightens his hold.

"We can't get separated," he explains, but she only offers him a baleful look. "C'mon Beckett, you're a believer in this stuff too, and if we are in another universe, than I don't want anything to happen to you."

She sighs, tugging on their tangled hands when the elevator doors slide open and allowing him to take the lead on the walk to his door.

"You're knocking to enter your own home?" she asks incredulously when he lifts his fist to the door.

"If all is normal and my mother answers, I'll just tell her I forgot my keys," he shrugs, knocking on the front door of his loft with three shorts raps of his knuckles.

Kate listens to the sound of soft footsteps growing closer on the other side, already assuming she knows they must belong to Martha, but when the door finally opens, she feels her heart stop.

No way.

"We are in an alternative universe," Castle squeaks, slapping a hand over his mouth as the woman who shares her face, her body, looks between the two of them with wide eyes and parted lips.

"Who is it?" another voice floats from inside the loft and then Castle, the other Castle, is standing behind her doppleganger's back. "Oh my god. Captain Beckett?!"

"From your dream?" the reflection of her hisses, spinning towards the man at her back while he nods eagerly.

"You found me!" he exclaims, flicking his vibrant blue eyes between her and her Castle. "And apparently I didn't actually die in your world?"

"Whoa, this is the me you met before you met me-me? The one who saved your life?" the man still holding her hand questions, excitement flaring in his eyes and is she - and the self in front of her - the only one actually freaked out about this?

"That's definitely me! It really wasn't a dream!"

Kate - the one who has a wedding band on her finger - pinches the bridge of her nose and tugs on Castle's - the one who wears a wedding band as well - arm and murmurs for everyone to get inside before someone sees them.

She doesn't let go of her Castle's hand, she won't lose him in this world too, and allows their two alternates to lead them inside.

"I like what you've done with the place," her Castle murmurs, appraising the loft with an appreciative nod.

"Much better than what you let your version of my mother do to yours," the other Castle huffs and she catches her reflection's gaze, shares a baffled look with herself before allowing her eyes to slip back to the band on her finger.

"You got to marry her," she says suddenly, feeling herself smiling softly before she can stop it, and the other Castle pauses, the one she met in what feels like a lifetime ago, taking the other Kate's hand while he nods.

"I have you to thank for that actually. After what happened in your world, I couldn't waste another second," he murmurs and his Kate tears her eyes away from the two of them to meet her husband's gaze, so much love consuming her face, it's almost sickening.

"I guess I should be thanking you as well then," the Kate of this world decides, glancing back towards herself with a hesitant grin that Beckett matches.

"This is all so cool," her Castle whispers, squeezing her still captured hand and she chuckles. "I have so many questions."

"But we have to figure out how to get home," she protests, unexplained panic suddenly seizing her gut, because what if - what if only one Castle can exist in one world at a time? And only one of herself? She doesn't want anything to happen to this Castle and Beckett, but what about them? What about-

But just before she can voice her fears, something blinds her, blue light stealing her vision, something loud like gunshots filling her ears and causing the hand in hers to go limp and no, no, not again-

"Kate, shh, you're okay," his voice drowns out the agony and she jerks awake, the claws of the nightmares slipping away as he pulls her closer. "I've got you."

She peels her eyes back, forcing them to focus on the man lying next to her on the couch, stroking the hair from her face.

"Castle?" she croaks, sitting up so she can see him in the darkness of her living room, so she can touch her hand to his face. "It was a dream?"

His brow furrows, but he doesn't let her go. "What was a dream?"

"You were - we had been - and there was-" she huffs, dropping her head back to his chest and curling a leg at his thigh. It had just been a dream. He was still here, still hers, only not yet. Because they were still waiting, weren't they?

"Did you lose me again?" he asks quieter and she exhales shakily against his sternum.

Castle rubs her back, keeping his hand between her shoulder blades. They've fallen asleep together during one of their innocent movie dates before and his presence should be enough to ward off the nightmares, but no matter how it happens, she always ends up losing him when she closes her eyes.

This was the first time she had entered another universe with him though, the first time she had been met with conjured up versions of their dopplegangers. But it still concluded the same way they all do - too abruptly and with him falling victim to a bullet.

She didn't want to waste any more time waiting.

It's not the first time they've kissed since they met again in a coffee shop only two months ago, but it's the first time she allows her want for him, for everything the other version of himself had represented, to pour from her lips onto his.

"Kate," he gasps, moaning low in his throat when she shifts above him, aligning their bodies so perfectly, fitting against him like a missing piece of a puzzle and god, she's even starting to think like him. All poetic and romanticized. He'd probably ruin her in every universe.

"I just want you," she whispers, the only explanation she can give, and he smiles against her lips, sitting up on the couch with her in his lap and rising with her still in his arms.

He's only just shut the door to her bedroom when he presses his lips to her ear.

"Already yours, Captain Beckett."