Castle had meant to propose the night before, but stuff got in the way- Alexis had called, and Ryan had shown up with some problem at the precinct. The moment had been ruined, and now he had a diamond ring burning a hole in his pocket.

On the way to the crime scene, he'd been tuning out what Beckett was saying, formulating a Plan B in his head. They could go out for dinner again tonight, he could reschedule the Hamptons trip. This could still work.

Unfortunately, the cold shoulder he'd been giving Kate while preoccupied trying to figure out a way to propose to her only got her mad at him, so when they showed up at the scene she'd frozen up on him.

"What've we got?" she said, greeting Lanie with a warm smile. Rick was looking around, the impact of where they were just now hitting him.
The Public Library. The first place Kate had cuffed him, the place they'd met after the mess with Jerry Tyson last year. Their place.

Lanie was saying, "Contusions on the left shoulder-" when he held out a hand, the other digging in his pocket.

"Lanie, stop." She glared up at him, brows raised, and Beckett turned with her hands on her hips.

"Castle, what's goin' on?"

"Kate, I, uh-" And he knelt on his knee. Kate, unbelievably, rolled her eyes, but she stopped when he actually pulled out the ring. "Kate, will you marry me?" Her mouth dropped open, as did Lanie's. Rick was just looking up expectantly around the shimmering diamond.

"Castle," Kate said finally, "we are at a crime scene."

"Yeah, I know," he said. "Will you marry me?"

"We are at a crime scene," she repeated. "And yes. But we are at a crime scene."

"Shut up, Kate, kiss him!" called Lanie from the background. Castle turned to glare at her. "I'll shut up."

Beckett did kiss him, and then she slid the ring onto her finger. A few bemused uniforms clapped, and then Kate and Rick stepped away from each other.

"Now can we be professionals?" said Beckett.

"Betrothed professionals."

She rolled her eyes, sure, but she also rolled them down to check out the ring as she knelt to examine the body, and she rubbed at it absently when she stood up, and for the rest of the day whenever she looked at it she'd remember arresting Rick upon meeting him, and she'd remember handcuffing him in the library and then saying yes to him in that same library, and she'd think, Well, it always would've been a crime scene.