A/N: Happy Castle Fanfic Monday! This little piece of crap was written for a prompt from the lovely Dia (fembot77 on tumblr/twitter). She posted a gifset of images from the scene under the overpass in 4x21 "Headhunters," with the comment: "When I was watching this scene, I really expected one or both of them to lose their cool and have the talk, right there under the bridge." And thus a plot bunny was born. I hope you enjoy it.

Castle leaned against a stone pillar, watching Beckett speak into her phone. He couldn't make out her expression in the dim light here under the overpass where Glitch's body had been found a few days ago, but her tone was brisk as she asked Ryan to trace all recent phone calls from the pay phone.

A sick feeling was gathering in the pit of Castle's stomach - had been growing there for about an hour now, ever since he had slunk back to Beckett with his tail between his legs and she had produced that file.

It had only gotten stronger a few minutes ago when he watched Beckett go toe to toe with Slaughter on his behalf. She had stood up to the maverick detective in a way that he, Castle, hadn't been able to - and put her own job in jeopardy in the process.

And then, after Slaughter had left, Castle and Beckett had fallen back into their old, comfortable pattern: running theory together. From the glint in her eye and the familiar prickling of the hairs on the back of his neck, he was pretty sure they had cracked the case. Together. Just like they had cracked so many others in the past. Damn it.

All this time, you had my back, his own words echoed in his head, and the ugly sensation in his gut solidified to the point where he could give it a name.

It was shame.

It came bubbling up his throat and out his mouth before he could stop it. "Beckett, what happened?" he found himself asking, just as she ended the call.

She turned halfway toward him, her eyes still in shadow. "Just what you said. Glitch used the payphone to call someone for help," she said coolly. "Ryan found the outgoing call. He's tracing it now."

"No, no, not that," Castle said, waving the case away. "I mean ... what happened with ... with us?"

In the dim light he saw Beckett's eyes flash at him, but when she spoke again, her voice was still low and controlled.

"You moved on," she said. "Got yourself a new partner, a new - a girlfriend."

"A girlfr-" he began incredulously. "No, that's not what..." Then the rest of her words began to sink in. "I moved on? Is that what you really think?"

She shrugged, her face still turned away.

"Beckett." He took a step closer, suddenly tentative. "I'm not, that's not what I'm doing. With Slaughter? Hell, who in their right mind would want him for a partner?"

She was watching him out of the corner of her eye, wary. It was dark, but he thought he could make out the sharp line of her cheekbone, the set of her jaw. He knew this. Whatever had come between them (but she lied, whispered his rebellious heart; but so did you, his conscience hissed back), he still knew the lines of her face, the play of muscles under her skin giving her away. She wasn't as calm as she was trying to appear.

"I don't want a new partner," he said slowly, and then, pushing the words past a suddenly tight throat, "do you?"

"No," she said immediately, vehemently, and then she huffed a little and turned her face away. But something in her tone brought him staggering forward, abruptly urgent, grabbing her arm, pulling her around to face him.

All this time, she'd had his back, even when he had been acting like…

"Is that really what you thought?" he demanded, pushing down his guilt in favor of irrational anger. "That I was moving on from, from you? Why? Why would I do that?" He stared into her face, searching for - he didn't know what. But all he saw there was a mask, the same mask she had been wearing for months now. Suddenly he wasn't at all sure he knew what it was covering up.

"Why? To protect yourself, by not taking any more emotional risks," she muttered, not meeting his eyes.

He snorted. "Really, Beckett? That sounds like shrink-talk. Are you a psychiatrist all of a sudden?"

"No," she snapped, and whatever was behind her eyes flared up into anger to match his, "but I've been seeing one. Doing just what I said I was doing, working on it. Trying to be better. And you? You ask me what happened? You're the one who changed, Castle!" Her voice broke, quavering desperately through his name, but she pushed on. "You're the one who pulled away. Running off with another detective, with another-"

She stopped herself abruptly, but he could see the shape of the words in the air. Another woman. Anguish crumpled Beckett's face and she spun away again, jerking her arm out of his grasp.

Now he was truly confused. His anger drained away as quickly as it had risen, leaving him with that nauseating lump of guilt at the bottom of his stomach and a slowly developing sense that he had gotten everything wrong, absolutely everything. "Beckett," he pleaded toward her back.

Another woman?

She was seeing a shrink?

What the hell had he been doing?

A truck thundered across the bridge over their heads, rumbling the pillars around them. For an instant his fanciful writer's brain produced an image of the whole structure collapsing, falling down around the two of them, burying them in the rubble of their inability to communicate.

He walked around her, to where he could see her face again. His shoes crunched on the pavement; the orange light of the streetlamps moved across him like sickly ghosts.

Beckett was braced for him by the time he came back into her line of sight. She lifted her chin and stared him down. But now he was determined to pull back that mask and find what lay beneath.

"You talk to your therapist about me?" he asked quietly. It wasn't the angle he had intended to lead with, but it seemed to strike home. Her head lowered defensively; she gave a little self-deprecating, humorless smile.

"Some days he probably wishes I'd talk about anything but."

"Kate..." He stepped closer, watching her eyes widen, her lips part soundlessly at the use of her first name. Taking another chance, he reached out and took her hand. "Why did you lie to me?"

Shock and fear flashed across her face, and something else - something new, something he hadn't seen in her eyes for a long time. It almost looked like hope.

"I was scared," she replied in a near-whisper. "I was broken. I wasn't good enough."

"Good enough for what?" He blinked, trying to put together everything she had said. He felt her hand trembling in his grasp. He covered it with both of his, running his fingertips lightly across her skin, and her gaze dropped down to watch the movement, but she didn't pull away. Didn't resist when he took another step closer. "For me?"

He watched her throat work as she swallowed painfully. "You got tired of waiting and moved on."

"No." He squeezed her fingers to emphasize the word. "I haven't gone anywhere, Beckett. I'm right here." He moved even closer, until their clasped hands brushed his chest. Her eyes were huge, staring up at him, shining with uncertainty in the hazy light here under the overpass.

"I don't want another partner, or another woman," he said firmly. "I don't care how scared you are, how much work you still need to do. I still love you, Kate."

She gasped at the words, and he felt the tremor that ran through her whole body, so close to his. "Castle," she breathed, as he watched a tear escape the corner of one eye and begin a slow descent.

He detached one hand from hers and lifted it to her cheek, wiping the tear away with his thumb. Then, almost in a daze, he found his fingers slipping into her hair, cupping the side of her head, gently tipping her face up toward him.

He leaned down, slowly, so slowly, waiting for her to tear herself away, to reclaim her hand and pull her mask back on.

But she didn't. She swayed into him, inviting, welcoming.

His lips touched hers and she let out a tiny sound, almost a sob, shot through with pure relief. He almost grinned against her mouth, because he felt the same way, oh god yes, the same way.

But there was no room for smiling, not when he had Kate Beckett in his grasp at last, and she was kissing him as fervently as he was kissing her, his hand still tangled in her hair, her arm sliding around his shoulders, their other hands still clasped together between their bodies.

The shrill sound of her ringtone broke through, bouncing off the concrete all around them, and she startled backward, out of his hold. They were both panting, staring at each other, dazed and dizzy.

Beckett, not surprisingly, regained her senses first. "Beckett," she snapped into the phone. "Yeah? Really? Okay, thanks."

She ended the call and looked over at Castle again, her eyes riveted to his face as she slipped the phone back into her pocket.

"Ryan traced the call," she murmured, her body gravitating slowly back toward his. "We know who the killer is."

"We should go then," he said hoarsely, regretfully. He was so mesmerized by her that it didn't even occur to him to ask the killer's name. He drifted closer to her, his hands coming up almost instinctively to rest on her hips. His heart leapt at the way she acceded to his touch, letting him tug her closer, tilting her face up toward his again.

"Yeah, we should," she acknowledged, just before he reclaimed her mouth. Her fingers danced across his cheek and jaw, and her tongue met his, but only for a moment before she pulled away.

"Come on, Castle," she said softly, taking his hand. "We've got a case to wrap up, and then ... we need to talk."

"Right. Yeah." He nodded, twining his fingers with hers, letting her pull him back toward her car.

They definitely needed to talk. And maybe it wasn't going to be pretty, but for the first time in weeks, his natural optimism was running the show again. It was all going to be okay. He was sure of it.

Beckett's car pulled away, out from under the bridge, and he didn't look back.