A/N: Hi, folks. This has been sitting on my hard drive forever so I thought I'd finish it and see what gives. Set during 4x11: "Til Death Do Us Part." You know the scene: "Are you really asking for my number?" ;)

"Castle, if we were getting married, would you want to know about all the guys that I've slept with?"

- Kate Beckett, "Til Death Do Us Part"

Slow Burn

Nothing else exists in this world after Kate Beckett utters the words, "Castle, if we were getting married⏤" with that open, adult look in her eye.

His hearing shorts out along with his brain, which is fried like green tomatoes. But the words remain stuck in a loop inside his head, echoing, so that all he can hear is, "Castle, if we were getting married..." on repeat, which quickly morphs into "Castle, we're getting married!" said in the same easy-breezy, coy tone she just used on him. As if this might be possible for them. Someday.

Kate is still talking; he can see her glorious lips moving. She's been so happy lately; happy with him, flirting, making him want things he's only ever admitted to wanting in the burning darkness of his bedroom. He thinks for a weird second that he just heard her tell him she's not a virgin, which is interesting, but he can't focus on anything she says as she stands in front of him right now while these words roll around and around his skull like a chanted mantra. "Castle, if we were getting married..."

Eventually, the slow burn in his heart builds, kindling that catches fire, and all he can think to say is, "Why aren't we?"

This pulls her up short. Her eyes widen and her jaw falls slack. From the look on her face, he's not sure whether Kate has no idea what he's talking about, or if maybe she does and she's appalled. She shakes her head slightly, as if she's clearing water from her ears, and her hair shimmers against her shoulders. The urge to reach out and touch her hits him hard, and he has to fight it with everything he has.

Kate's eyes are fixed on his, such beautiful eyes, and then she tilts her head slightly to the side and narrows her gaze. "Why aren't we what?" she asks warily.

Castle notices how her fingers twitch against her thigh as if she's holding herself back from reaching for something. So he clears his throat and takes the biggest, dumbest, bravest gamble of his entire life. "Why aren't we getting married, Kate?"

"Shit. I thought that's what you meant," she says in a hot rush of words. But instead of laughing or maybe slapping him, she's closing the door, and when she turns around she's smiling. Just faintly, but it's definitely there.

"Is this wedding fever?" she says sympathetically, regarding him with a look of amused tenderness. "Ryan and Jenny's big day making you all sentimental? Nostalgic? Because, Castle, you have quite the jacket in that department. Two priors, in fact. Don't think the DA's gonna let you plead out on the next one," she jokes, shaking her head with this gentle, smiling scold. "Nope. Three strikes mean you're in for some serious jail time," and she laughs.

"Next one's for keeps," he blurts recklessly, and they both freeze. There's no suave, no wooing, no elegant, slow burn now. His throat is dry, his voice rough as gravel, but it works, throwing her off balance if the dilated pupils and that faint blush creeping up her neck are any kind of a clue. She's stopped laughing, too.

Kate takes a step closer. He can feel the heat coming off her body and see her eyelashes beating out a flustered tattoo like she's trying to put out a fire that her eyes keep reigniting. "Are you⏤ Were you being serious just now?" She frowns, her voice a little panicky at the prospect of his answer because the man is nothing if not honest.

Castle licks his lips, juts out his chin, broadens his chest, and says, "Deadly." His heart is beating violently and his palms are sweating, but, man, he is proud of himself.

Kate gasps her next breath of air and says, "So this isn't a prank? To be clear?" He shakes his head. "And it isn't some…oh, I'm over forty and I'm getting lonely and all my friends are⏤" She stops mimicking him when she sees that he's still shaking his head, calmly. His mouth has formed a firm, straight line, and his eyes shimmer with a mix of sincerity, worry, and caution-to-the-wind optimism.

Now Kate looks nervous, her brows creased. She's worrying her lip between her teeth, and goddamn if that isn't just the sexiest thing. "Did you just propose to me?" she asks breathily, and the second the words are out of her mouth, Castle wonders what the hell took him so long. Not to have done so before now seems like utter insanity. Of course! It's as if someone just explained his mother's beauty regime or cryptocurrency, and suddenly all the songs make sense.

"Yes." He nods bravely, gathering strength. "Yes, I did." Dramatically, he drops to one knee, and Kate kind of squeaks. When he looks up at her, there are tears in her eyes. She presses her fingertips to her lips and holds her breath as he says, "Katherine Houghton Beck⏤"

Before he can finish, the break-room door flies open so hard that it hits the wall and rebounds with a shiver. Esposito bursts in and skids to a halt in front of them. "Yo, Castle! Beckett got you cleaning floors with your tongue now?" Espo cackles at his own wisecrack, but no one else is paying him any attention.

Castle stumbles to his feet. But by the time he's upright, Kate is gone. He closes his eyes, drops his head into his hands. This cannot be happening.

TBC...