for carolina17 who wanted a story named Catalyst. it's not exactly what we talked about, but maybe it will do?

Catalyst

a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

He didn't mean for it to happen. He meant to have them stumble close enough to the lookout on guard in front of the warehouse so that Kate could take him out cleanly, quietly. He meant to smile and laugh with her, crookedly walking down the street, until they were close. He meant to have her against him, his arm loose around her waist, smelling her hair, and maybe let his fingers innocently stroke her hip.

What he meant was, it went differently in his head.

What happened, instead, was that his idea came off the way most of his stupid ideas came off: poorly. The guy was so not buying it, Kate was tense and ready to spring even while she played at sloppy and slurred, and Castle himself felt like his act wasn't to be believed.

She was giggling, she was moving around him to get in a better position, she was twisting her arm back to get under her coat for her gun and that would be a Mexican standoff, that would be bad, someone was going to get shot.

So he did the only thing he could think of. He panicked. He knocked her hand away from her gun, and grabbed her, hard.

And everything was so clear. What he had to do, what lay ahead of them, the way this would work out in the end.

He stared her down, telegraphing his intent, and waited until the fake giggle died on her lips and her eyes met his, brown and dark and fathomless, but full of knowledge.

And he kissed Kate.

Thoroughly. Gently.

With his hands at her cheeks, his body crowding into hers, he made a case for himself with as much tenderness as he could manage, even while his lips apologized for it.

He held her there, before him, and worked his mouth over hers completely, warm and moist and rich. She took it.

She pulled back first. A jerk of her head but not her body, so that even while she stared at him, stunned, her lips wet with his kiss, she didn't go far. She breathed, she looked at him, struck dumb, and as he waited for something, anything to happen, she swayed on her feet like she was going to fall into him. At the last second, her eyes flicked over his shoulder.

And then she was on him. Not just lips and heat, but tongue and teeth, taking from him, pulling herself up into his body, clinging, her kisses fierce and unapologetic and sexy as hell. He slid a hand up her back, pulled her closer, a hand in her hair to anchor her against him, never let her go, and she sidestepped to press her hips against his thigh, he was ducking to get at her lips, another taste of her, her hand at his neck tugging him in-

And then she was unlocking their lips, pushing back from him, a roll of her body outward, in flight, the grunt as she left his lips, lonely, now mingling with the grunt of the man as Kate's kick dropped him where he stood.

She was panting. He could see her shoulders heave. Her hair still tumbling down her shoulders.

Gorgeous.

"That was awesome."

When she does turn to look at him, slowly and in control, still breathing hard, her face is questioning, but her mouth is still partially open, wet, inviting.

He fumbles his way through an explanation; Kate, still breathless, moves ahead of him to after Esposito and Ryan, all business. If it weren't for the smudged look of her lips, he'd have no idea she had just shared one of the hottest, most intense kissing experiences of his life.

She was in control again.

He wiped his hand against his mouth, shivered, and followed her inside.

Catalyst

a person or thing that precipitates an event

When the ambulance arrived, Esposito and Ryan made a show of shrugging off the paramedics, and Castle, because he was both in pain and also participating in the maintenance of the boys' machismo, cornered the EMT about his hand. The guy looked back and forth from Ryan, who was still soaked to the skin and bloodied, and Castle, who was wincing and sucking on his knuckles, and then sighed and led Castle over to the back of the bus.

He sat down inside as the paramedic slowly probed the bones of his hand, running his fingers down the long metacarpals, working the joints. Rick hissed, wincing with his shoulders up around his neck, until the EMT diagnosed a few cracked knuckles. "I'll wrap the hand, give you a chemical ice pack, and then just keep it on ice for twenty to forty minutes at a time."

The paramedic did a quick, tight job, then left to get the ice pack from his bag still out in the middle of the street. Castle winced, his hand throbbing with pain. It hurt worse now that it was wrapped.

He started unwrapping the white ace bandage and saw Beckett appear on the bottom step, then hop up into the back of the ambulance across from him.

"Hey there, Chuck Norris," she said, her voice amused.

He chuckled and tried to figure out the wrap, his mind still somewhere back in that warehouse, watching as Kate skulked through the loading equipment, Lockwood's sights on her. Lockwood had been about to pull the trigger. And then he jumped the asshole and beat the shit out of Lockwood, furious and pissed and sad. Somewhere in him, he realized that he'd been broken tonight, that something in her face after that kiss had left him a little hopeless.

Whereas Beckett was all business as usual.

And that made him sad too.

She smiled at him as he tried to loosen the last of the wraps. "How's the hand?"

And to his surprise, she took hold of the end of the bandage while he fumbled for an answer, coming up only with the truth. "Uh. . .excruciating." He huffed out a breath and watched her slowly rewrap his hand, her fingers gentle and light against his skin. Who knew that just the light touch of her hands could be so erotic?

"How's Ryan and Esposito?" he asked, because he felt like he had to say something, distract him from the clawing need that demanded he kiss her again. Here, now. Take her.

"Eh," she murmured, her eyes on her task. "Mild hypothermia. Wounded pride. Guess which one will heal first?" She smirked and lifted her eyes to his, and he was breathless for a second, struck by the rich chestnut of her hair and the dark delight in her eyes.

This was more than job well done, Castle.

He smiled back as she tied off his wrap, her hands stroking across his before letting him go. He held his hand against his chest and realized the wrap now smelled like her. It still throbbed mercilessly, but, well, it felt better too. All better.

"Thank you," she said softly, bent over with her elbows on her knees and her face both serious and joyful. Such joy in her eyes. When had that happened? "For having my back in there."

Their knees were so close that he could feel her heat; the heat of certainty, and thirst, and mystery.

"Always."

This joy wasn't about getting Lockwood; it wasn't about all of them coming through this alive. The joy in her eyes had something to do with *him.*

He had made his case. He had gone into danger with her, as he always would, and he had proved himself. Had the cracked knuckles to show for it. So maybe she was just a hell of a lot better at being professional, but she was not unmoved.

He grinned back and leaned forward, snagged her hand with his uninjured one, and brought her fingers to his lips.

"Partners," he promised.

In any way you'll have me.