A/N: Set any time between 4x10, 'Cuffed', and 4x11, 'Till Death Do Us Part'.

"This is a bad idea," Kate whispers as Lanie fluffs the skirt of her dress.

Her best friend straightens to adjust the stray curls of Kate's hair, ensuring the pretty bun at her nape is secure.

"A little late to be getting cold feet, honey," Lanie says under her breath. "Besides, I don't know why you're even freaking out. It's a fake wedding to lure out a newlywed obsessed serial killer. Sounds right up your alley."

Beckett huffs as the wedding march begins to play and Lanie shoves the bouquet into her hands.

"C'mon, girl," Lanie reasons, moving to stand in front of her before the double doors can open to reveal her. "I know you can kill an undercover mission, especially when you've got your boy as your backup."

That was the problem. When she and Castle go undercover, it tends to go too well.

And always seems to end with his mouth on hers.

It's a guaranteed part of today's mission and it doesn't end there. She has to unofficially marry him, continue the ruse through a brief reception at the hotel he too generously volunteered to book for the event, and then they have to sneak away to their honeymoon suite for the night.

Their wedding night.

Her hands won't stop shaking.

Kate sucks in a breath and clutches the bouquet as the doors to the church their suspect has been using to scout for couples are pulled open and the congregation of actors hired by the owners of the venue turn to place all eyes on her.

But she automatically searches for him.

At the other end of the aisle with Ryan as his best man, she catches a glimpse of Castle over Lanie's shoulder, wearing thick, black framed glasses to make his identity a little less obvious and a crisp black tux that she knows must fit his frame well, accentuating the breadth of his chest and the strength in his shoulders.

His body built muscle over the summer, has grown visibly firmer in certain places, not exactly making the process of 'waiting' any easier. But she can't think about that now, can't think about the wall or how badly she already wants it to come down, about the way he looks at her every morning when he presents her with a cup of coffee, about the bullet scar currently itching between her breasts.

She's walking down the aisle, about to fake marry him for the sake of snagging a killer's attention, and she needs to focus solely on that. But when she and Lanie reach the front of the church and her maid of honor steps to the side, offering her a clear view of her 'fiancé', the expression on his face, the pure awe in his eyes… she knows she's screwed.

Castle holds out his hand to her, dialing back the adoration that claimed his face, trading it in for a warm smile that soothes the skipping of her heart. She grips his fingers and moves to stand with him at the altar, before a church full of strangers, three of their friends, and the handful of undercover officers blended throughout the meager crowd.

"You look beautiful," Castle breathes, sweeping his eyes over the simple but elegant dress Lanie helped her find last minute and within Gate's enforced budget, even though Castle repeatedly offered to give her his credit card. "Kate-"

The pastor officiating the ceremony clears his throat. Castle zips his lips while hers quirk with an overdue smile.

"Thanks, Castle," she murmurs, squeezing the hand in hers as the officiant begins to speak, putting on the usual show, one he isn't aware is all a fraud this time.

"And now for the vows," the older man announces, nodding to Castle first.

Her heart flutters with nerves, with something more, as he withdraws a piece of paper from within his jacket, hesitating as he stares down at the notecard. They both had the night to prepare their vows, but the index card she glared at all evening remained blank. There's only one word he would need from her anyway.

But Castle crumbles the vows written like a script and meets her eyes as the paper crunches in his palm, gazing at her like she's the only person in the room.

"Kate, the moment I met you," he begins, his voice quiet, subdued, as if the words really are meant for her and her alone. "My life became extraordinary."

She knows before he even continues that there's no way she's surviving this.

"You've taught me more about myself than I ever thought there was to learn. You're the most remarkable person I've ever met, the most maddening and challenging." His lips quirk and he strokes his thumb over one of the fingers still tangled with his. "You're the joy in my heart and the mystery I want to spend the rest of my life exploring because I... love you, Kate Beckett, and-"

She isn't thinking when she steps forward, one of her hands still encased in his while the other ascends to his cheek, cradles it in her palm as she seals a kiss to his lips.

Castle goes rigid beneath her, a quick intake of breath causing his chest to hitch against hers, before he covers the hand at his cheek, strokes his thumb over her knuckles.

"Excuse me," the priest hisses under his breath. Kate startles, almost curses herself aloud, and jerks back from Castle. "I'll take that as your vows, Miss," the older man mutters, casting a long suffering look to them both. She can feel her flesh on fire, mortified and a little shocked by her own actions. But Castle is watching her with awe in his eyes, soft and breathtaking and just shy of too much. It horrifies and helps her at the same time. "Now, if you'll just exchange the rings, you may resume in your kissing of the bride."

Castle grins and produces the simple gold band within seconds, but it takes her a moment, with her trembling fingers and overheated cheeks, to fish hers from the small strip of fabric at the waist of her dress.

She kissed him. Not for the sake of the ruse, not even at the right time, and - and shit, everyone saw and Castle probably thinks-

Rick lifts her hand, slides the ring onto her fourth finger, brushes his thumb over the spot once the jewelry is in place. She meets his eyes with the tentative rise of hers, finds him staring back at her with a calming shade of blue that reminds her of the ocean, reminds her to breathe. Beckett mimics his actions, steadies his left hand in hers, and eases the ring onto his finger.

"Finally," the priest mumbles under his breath before pasting on a smile for the congregation. "By the power vested in me by the state of New York, I now pronounce you husband and wife. You may now kiss your bride. Again."

Their audience rumbles with quiet laughter that Kate can hardly hear over the rush of blood through her ears. It's far more nerve-wracking this time, knowing what's coming rather than acting on impulse.

Castle drapes his hands at her waist while hers curl in the lapels of his suit, and then he's leaning in, whispering a kiss over her lips that is hardly a kiss at all. Hardly convincing of a newlywed couple who are supposed to be madly in love.

Hardly matching the kiss she pressed to his mouth only moments before.

Kate tugs on his lapels, swallows his gasp as she kisses him firmly on the mouth, smirking as he tightens his grip on her hips.

That earns them the applause, the wolf whistle that she just knows has to belong to Esposito at the back of the church.

When Kate withdraws, she has to press two fingers to his chin to stop him from chasing her mouth.

"That was better than my first two weddings combined," he breathes into the space between them. Kate rolls her eyes, but her skin is on fire all over again.

After the ceremony, they convene with the boys in the bride and groom's white limo on the way to their fake reception.

"I'm going out on a limb here, but I think you guys were good enough to hook our guy's attention," Ryan announces as they all settle into the back, the driver an officer from the Twelfth and already maneuvering through traffic to start towards the nearby hotel Castle and Beckett will be staking out in for the night.

Esposito smirks, loosening the bowtie at his neck. "Yeah, those were some vows, Castle. Made Beckett break character and everything."

"Excuse me?" Beckett interrupts, arching an eyebrow at the two of them while she adjusts the lace embroidered sleeves of her gown. "I never broke character. What bride would have just stood there during those vows?"

She watches their confidence waning under her reasoning, feels Castle's amused gaze flicker towards her, but he won't say anything, even if he does know the truth.

"It was either kiss him or cry and I'm not one for tears," she shrugs. "Now, are we going to discuss the home stretch of this assignment or did you guys want to keep gushing over Castle's way with words?"

"I am not opposed to either," Rick pipes up, sipping from a glass of the champagne provided for them.

Both of the boys grudgingly straighten, Esposito muttering under his breath how he knew all along.

"Okay, so the guy apparently favors the honeymoon suites of the hotel the church has a contract with. Or, at least, that's been his M.O for this long, but I don't see it changing. Our perp is all about rage, not strategy."

Beckett's listening to Esposito as he goes over their killer's profile, but her mind is also wandering to that damn honeymoon suite they're going to be stuck in, the potential hours she could spend locked in there with Castle. Their suspect has never struck at one specific time, Lanie discovering varying times of death amidst the three couples that have been killed thus far. All Beckett knows with certainty is that they've moved past being doe eyed newlyweds and onto lovers about to experience the oh so magical wedding night.

She doesn't think she's going to survive this either.