In retrospect, Kate Beckett realized that she had missed the first clue: it was too quiet.

Sure, part of that was because Martha was at a rehearsal and Alexis was out with friends, but she should have noticed the prolonged silence and the way that the world seemed to collectively hold its breath. Her well-honed instincts should have kicked in and warned her to be on the alert.

Maybe she'd been away from the precinct for too long.

Or maybe she'd just gotten far too engrossed in the latest James Patterson novel.

So when Richard Castle tried to tiptoe behind her, she didn't look up. In fact, she didn't even notice he was in the room until a loud pop and a sploosh startled her right off of her perch on the couch.

"What the hell, Castle?" she demanded. She scrambled to her feet, glaring at him as she tried to calm her racing heartbeat.

Castle froze in place and stared straight ahead with wide eyes. Busted. "This is not what it looks like," he pleaded as he finally turned to meet her fiery gaze.

Beckett took in the whole scene and suddenly wasn't sure whether she should laugh or smack him upside the head. In his arms, Castle was balancing a whole load of water balloons. "I'm sure it isn't," she said evenly. She fixed him with the same challenging stare that she'd given so many suspects as they sat across a table from her.

"I was… this is… I can explain!" When she only raised an eyebrow, he stammered, "I thought it might be fun… like in college… oh, come on, it's summer!"

Beckett's eyes narrowed. "You were going up to the roof, weren't you?"

"Yes… no… maybe?" Castle pursed his lips and looked away sheepishly. Another balloon fell off his precarious load and splattered on the floor.

Finally, Beckett allowed the smirk that she'd been hiding to slip across her face. "Trying to get yourself arrested again, huh? Disorderly conduct?"

"You wouldn't turn me in, would you?" he asked with an exaggerated pout. "I mean, it is positively scorching outside today, and wouldn't I be doing everyone a favor by cooling them down?"

"From the top of a city building? Somehow I don't think they'd see it your way." When Castle continued to pout, Beckett remained silent for a moment. Her eyes flicked from the remnants of the two fallen water balloons, to the load in his arms, then back up to his face. "How about we restrict our battle to the rooftop itself?" she replied slowly as a smile tugged at the corner of her mouth.

Castle's face lit up. "Really?"

"Really," Beckett replied, rolling her eyes.

"Okay, help me get these into a bucket, so I can go grab the rest!" Castle bounced eagerly on his toes and for a moment Beckett thought he might lose yet another balloon.

Laughing, she wandered to a nearby closet, pulled out a five-gallon bucket and carefully emptied his arms. "Where are the others?" she asked as she set down the last balloon.

"In the bathroom," Castle answered. Beckett picked up the pail and brought it with her as she headed off to collect the rest. He trailed just a few steps behind.

Moments later, he ran into her back when she stopped suddenly, just inside the door to the bathroom. "Castle!"

"What?" he asked innocently.

Beckett gestured inside. "You filled the whole bathtub with water balloons?"

Castle huffed. "Come on, everyone knows that the biggest summer mood-killer is when you run out of water balloons. I was just being prepared!"

"Fine. But you get to carry them all up to the roof. And Castle?"

"Yeah?"

"I think we're gonna need a bigger bucket."

By the time they stepped out onto the roof, Castle's shirt was already completely soaked. "No fair," he whined. "You already have the advantage now."

Beckett set down the wheeled cooler and raised a teasing eyebrow at him. "How is that not fair? You were the one who thought it would be a good idea to bring up the last of the water balloons in a backpack."

"But I didn't think we'd have to share the elevator with Mrs. Doheny, her baby carriage, and her three rambunctious boys!"

Beckett shrugged unsympathetically and shot back, "Always expect the unexpected, Castle."

When she leaned down to take the lid off of the cooler, a sudden, wet slap hit her square in the back. She gasped and whirled around to see Castle staring her down, a triumphant smirk on his face. "Always expect the unexpected," he mimicked.

"Oh, you are so going to pay for that," Beckett growled, reaching for the cooler again.

"Game on," responded Castle.

With that, they dove for the balloons and then took off across the roof.

WriteRCastle: "Celebrating the first day of summer by filling up 120 water balloons and heading to the roof."

WriteRCastle: "Elevator mishap. I'm very, very wet."