A/N: I have no idea where this came from, all I know is that it wanted to be written and it was composed and posted quicker than I would normally allow, but figured I should before I lost the nerve, so apologizing ahead of time for potential mistakes.

Set in late season 3, before the finale but after 3x22 (To Love & Die in L.A)

First M rated fic, so be kind, and hopefully enjoy, while I go hide under a rock for a bit?

What are you doing?

The phrase ran through her mind on a loop as she slipped out of bed and tiptoed into the bathroom. It was 3 a.m. and her body was thrumming almost painfully with a need only one person could fulfill. If only she would let him.

She shut the door soundlessly behind her and bit her lip while she drew herself a bath, praying Josh was exhausted enough from his nonstop shifts at the hospital to sleep through the sound of running water in her en suite bathroom.

He had come over thinking she would be overjoyed to see him after being apart for nearly a week because of both their busy schedules, but instead she'd had to fight a frown and force a smile as she had ushered him inside her apartment, shared a pizza with him on the couch, and then crawled into bed with him only an hour later. He had curled up behind her almost immediately, tracing his hand along the hem of the oversized t-shirt she wore to sleep in, but she had shrugged him off, claiming she was too tired, and he had bought it all too easily. He always did.

Nearly four hours later and she was still wide awake and her body was a livewire humming with electricity.

She wanted to call Castle. It wouldn't help, it would probably only make things worse, but she wished she could have his voice to surround her. Maybe it would help soothe the never-ending restlessness that inhabited her, at least for the night.

Ever since they had returned from LA, the always underlying tension between them had been amped up a notch. Instead of the typical but bearable want, the controllable lust she harbored for him, she had started to crave him. Dreams, fantasies, indecent stares aimed at the broad expanse of his chest that lasted too long when he wasn't looking – all becoming daily nuisances for her to tamper down and she was mentally exhausted from it.

She sighed and shook her head watching the bath water rise. She was acting like a horny teenager and it wasn't like her. They had worked like this for three years, why does one night in a hotel room across the country change anything? Nothing even happened.

Kate shed her shirt and panties and stepped into the half filled tub, letting the water continue to stream in and exhaling contently as the warmth of it embraced her shivering form. The relaxing lounging in the bath didn't necessarily cool the desire buzzing underneath her skin, though, and she soon found her fingers trailing absentmindedly along her chest, teasing her breasts until her nipples were puckered and hard, and then tripping down her abdomen and hesitating momentarily before slipping into the encompassing heat of her folds.

She tipped her head back against the rim of the porcelain tub and closed her eyes, seeing the ice blue pools of Castle's flashing back at her behind her lids.

Her chest hitched at the vision, her breasts rising up and out of the water as her back arched.

He would fit well in her tub. It was small, but it was still big enough for two and she could practically feel the heat of his chest against her back as she imagined him settled behind her.

As her fingers slid into her entrance, she pictured his thick hands replacing her thin ones, circling her clit with his thumb and filling her better than she ever could herself with two of his fingers while his other hand massaged her breast. She clamped her teeth down on her bottom lip to keep from moaning aloud as her hips jerked without her consent and sent water splashing out onto the floor. She'd forgotten to turn the tap off and it was getting too high, swelling up to pool around her collarbone now, but his fingers were inside her, plunging and curling in all the right places while his thumb worked her overly sensitized clit and she just couldn't stop long enough to reach for the nozzle, not when she was already so close so fast.

She grit her teeth and planted her feet flat against the floor of the bathtub and curled her unoccupied hand around the rim to keep herself from slipping as she thrust into her own hand - his hand - and furiously rubbed her clit until she was panting from the exertion. At last her body seized against her ministrations and the tight coil in her stomach finally unfurled into a wave of white hot pleasure that rushed through her limbs, flushing her out and leaving her a quivering mess in a tub of too much water.

Her body felt better, so much better, but her conscious began spiraling even deeper into despair as she lay there floating and sated, asking her difficult questions. Like, why had she gotten out of bed, a bed that held a perfectly willing and capable boyfriend, to get herself off with the illusion of another man's hands on her. Accompanying guilt made her ask herself what kind of person that made her. Aside from one who didn't love her boyfriend.

She had bit her lip so hard she'd nearly drawn blood to muffle the moan of Castle's name falling from her mouth when she had come undone from his imaginary fingers between her legs.

The worst part, she thought, was that she found release far more often from someone who wasn't there compared to the man currently sleeping in her bed, who she hardly allowed to touch her anymore. Josh had never given her an orgasm like the one she had just experienced alone at 3 a.m. in her bathtub. She had a feeling only one person could.

She waits until her breathing has returned to normal, turns off the water before it can spill over the edge of the bath, and reaches for the plush white towel hanging on the opposing wall.

She spent the rest of the night mopping up the spilled sea of water surrounding the tub.