Author's Note: I really thought I was done writing Castle fanfic, but then ladyalways47 posted a fic prompt, and Twitter informed me that today is National Donut Day, and somehow this happened. It's pretty ridiculous. Enjoy!

"Guys! Guess what?" Castle exclaimed as he stepped off the elevator and approached the bullpen of the 12th Precinct.

"Guess?" said Esposito, scowling. "We're detectives. We don't guess. We detect."

"Okay," Castle said, undeterred, "then detect what?"

Esposito scoffed and crossed his arms over his chest. "I detect that you brought donuts."

"Not just donuts," Ryan corrected his partner. "I detect Blackbird donuts." The shop's logo was prominently displayed on the three large white boxes Castle was carrying.

"And I detect that you already ate one," Beckett put in with a smirk. "A powdered, to be specific." She leaned back in her chair and quirked an eyebrow at her boyfriend.

Castle glanced down at the dusting of powdered sugar that coated the front of his blazer, and grinned sheepishly. "Yes, okay, fine," he said, "you're all excellent detectives, a credit to the NYPD." He carefully set the boxes on Beckett's desk and brushed the incriminating sugar off his chest. "But what you failed to detect, gentlemen and lady, is that today is National Donut Day, and in honor of same, Blackbird Donuts has introduced a new product."

With a dramatic flourish, he reached over and flipped up the lid of the top box. Ryan and Esposito leaned closer to peer inside. Beckett just watched, shaking her head and biting her lip to hold back a smile.

"Mini donuts?!" Ryan exclaimed, his eyes lighting up. Beckett sat bolt upright in surprise. Heads turned throughout the bullpen. And Castle beamed as proudly as a new father.

"That is correct, sir. The finest mini donuts money can buy, at least in the Tri-State area." He grinned at Beckett. "And if I'm not mistaken, mini powdered are your absolute favorite, Detective."

Beckett peeked into the box, then looked back up at Castle again. "I'll thank you later," she murmured in a tone of voice that made Ryan blush and Esposito wince.

Castle's grin widened. He glanced over at Captain Gates's office, making sure it was dark and empty. In the weeks since the incident with Beckett standing on the bomb, the captain had made good on her promise to pretend that she didn't know about their relationship, and they had held up their end of the bargain by refraining from PDA in the precinct. Mostly.

Gates wasn't there at the moment, but Ryan and Esposito were - not to mention the horde of hungry uniformed officers currently eyeing the white boxes from around the room - so Castle confined himself to a light brush of his fingertips across the back of Beckett's hand. "I'll look forward to that."

Ryan cleared his throat discreetly (or so he thought) and reached out to move the top box aside so he could look into the next box underneath it. "Ooh, there's mini chocolate ones too!" He took one out and beamed at it. "They're so cute."

Esposito nudged his partner aside and moved the second box so he could look into the third. "Plain and cinnamon," he observed. "Good choices."

"I'm glad you approve. Help yourselves," Castle said. Seeing Beckett's covetous glare, and the way her hand landed possessively on the lid of the first box, he added, "Not to worry, Detective. I had them set some aside for you in advance." From his pocket, he produced a wax-paper bag, which he handed over to Beckett. She opened it, looked inside, and smiled.

Within moments, Ryan and Esposito had taken the boxes of donuts to the break room, watched avidly by every other cop in the room. Beckett took one mini powdered donut out of her bag, carefully tucked the rest away into the bottom drawer of her desk, and took a delicate, tidy bite. Castle watched with a self-satisfied grin.

"Stop looking so smug and get me a coffee," Beckett instructed, not looking at him. Her gaze remained fixed on the tiny donut in her hand.

"Yes, ma'am," Castle said cheerfully, and proceeded into the break room, where he accepted back-slaps and mumbled thanks from the rapidly growing crowd of cops.

By the time Castle had constructed a cup of coffee the way Beckett liked it and brought it to her, the three white boxes in the break room contained little more than crumbs, and Beckett's mini donut was long gone. A peaceful quiet descended once more on the 12th Precinct as cops returned to work.

Castle settled himself in his usual chair next to Beckett's desk, looking at her hands as she tapped away on her computer keyboard, filling out a warrant request for their latest suspect's phone records. "How do you do that?" he asked.

"Do what?" Beckett asked absently. "The warrant request form is pretty straightforward. You just put the case number here and-"

"Not that," he said. "This." He gestured to Beckett's blouse. "You ate a powdered-sugar donut and didn't get a single speck of powdered sugar on yourself. It's not natural."

She pressed her lips together to hold back a smile. "Sure it is. It's a natural talent."

Castle's eyes twinkled. "And here I thought I'd already discovered all of your natural talents."

She slid her gaze sideways to meet his eyes, just briefly. "I'm not responding to that in the workplace, Castle."

"Right, right. Sorry." But he was still grinning. "I love you, Kate."

"Shut up." But she whispered it back under her breath. "I love you too."

Warmed by the words, Castle obediently shut up. He sat quietly and watched as Beckett continued typing.

His gaze was on her hands, but then it drifted down toward the lower drawer of her desk. Then back up to her hands again. Now he looked more carefully, considering. A crazy, wonderful idea was forming in his brain.

Abruptly he jumped up out of his chair, startling Beckett, as well as the other nearby detectives. "Be right back!" Castle exclaimed, and dashed toward the stairwell.

"What's that about?" Ryan asked.

Beckett shrugged. "Who knows?"

"Better not be more donuts," Esposito opined. "Sugar crash is gonna be ugly already."

Within a minute or two Castle was back. "Kate," he said, slightly breathless. "I need to borrow one of your donuts."

"'Borrow'?" Beckett repeated in a tone of extreme skepticism, eyeing him. "What are you up to?"

"Please." Castle broke out his best puppy-dog expression. "Just one. Just for a minute."

Beckett's eyes were still narrowed, lips pursed, but she opened her desk drawer and took out a single mini donut from the bag. Castle accepted it from her with a dopey grin.

"Thanks. Have I mentioned how much I love you?"

"Castle!" she hissed, looking over at Gates's office again. Still empty.

Holding the donut in one hand, Castle grabbed Beckett's left hand with his other. He held her fingers up toward his face, examining them carefully. She stared at him in bemusement.

"The hell's he doing?" Esposito asked.

"Beats me," said Ryan, although in fact he thought he might have an inkling.

"Castle," Beckett began, but before she could chastise him further, he turned toward the bullpen at large, still holding her hand in one of his and the mini donut in the other.

"Everyone, can I have your attention for a moment please?" Castle said loudly. Beckett cringed and tried to pull her hand out of his grasp, but he held firm as the other cops in the bullpen turned to look.

"What are you doing?" she stage-whispered fiercely. Castle ignored her, addressing the room again.

"As you all know, today is National Donut Day," he announced, "and you've all enjoyed the mini donuts that I brought in. Now, this might sound silly, but National Donut Day has made me realize some things." He looked at Beckett, his eyes twinkling once again. "For one thing, I've realized that Detective Beckett is much better at controlling her powdered sugar than I am." A soft laugh rippled through the room.

"For another thing..." Castle continued, and in a surprisingly graceful motion, he sank down onto one knee in front of Beckett.

The assembled cops gasped and exclaimed, but Beckett didn't hear any of it over the pounding of her heartbeat in her ears.

"Castle, what the h-" was all she could manage before her throat mysteriously closed up. She swallowed thickly, staring down at him.

"Kate, I love you," he said steadfastly, "and I don't care who knows it. You're the most amazing, fascinating, frustrating person I know, and I love the way you love mini donuts."

A laugh startled out of Beckett and she lifted her free hand to her mouth, blinking hard. "Rick..." she whispered.

He held up the mini donut and smiled sweetly. "Katherine Beckett, will you marry me?"

The entire room held its breath. Tiny motes of powdered sugar flaked off the donut and drifted serenely toward the floor.

"Yes," Beckett breathed at last. She took her hand away from her mouth and said it again, louder and steadier. "Yes, I'll marry you."

Beaming, Castle slid the mini donut onto her finger as the gathered cops burst into applause. To Beckett's amazement, the donut was a perfect fit. Castle surged to his feet and swept her into a passionate kiss, setting off a round of whoops and hollers that turned Beckett's cheeks bright red.

When they surfaced from the kiss, there was powdered sugar all over one side of Castle's face, and Captain Gates was standing just outside the elevator doors, watching the commotion with an inscrutable expression. Catching sight of her, both Beckett and Castle froze, neither daring to speak as they waited for the captain to lower the hammer.

She took a long beat, studying them, and then at last: "Congratulations. Someone clean those donut crumbs off my floor."

"Happy National Donut Day, Captain," Castle proclaimed with a wide grin toward Gates's back as she proceeded into her office.

Beckett blushed fiercely, but she couldn't hold back a smile of her own as she studied the smashed mess of donut and sugar paste smeared around her ring finger. "Oh, what the hell," she said. "Happy National Donut Day, Castle." She flung her arms around him. "I love donuts. And I love you."