Title: The Exact Colour of Doubt

Author: shred01

Spoilers: All aired episodes.

Summary: Beckett goes undercover and Castle doesn't like it. Set in season 5.

A/N: All mistakes (spelling or otherwise) are my fault.

Disclaimer: Castle, sadly, doesn't belong to me. Maybe one day. Till then, I'm just pretending.

Castle didn't like this. He didn't like this at all.

In the tight confines for the NYPD surveillance van, Castle watched from his seat at the back as Esposito mic-ed up Kate. She was wearing that black dress that he liked. The one that fell to the floor and had a slit up to her left thigh and showed off her slender shoulders and back. The one that had a neckline that left little to the imagination.

Her hair fell forward, framing her face as she bowed her head and spoke softly to Esposito, directing him on where to go with the wires. The other detective was kneeling in front of her, wires in his hand, carefully threading them along the inside of her right thigh. Epso's face was furrowed in concentration as he took extra care in not letting his fingers slip or touch anything he shouldn't touch as he taped the wire to her skin. His eyes were focus solely on the task at hand and nothing else. He looked at little uncomfortable, knowing that Castle was watching him like a hawk. One wrong move and Castle would be on him. Castle didn't fail to let him know that.

"I'm watching you," he said. "You look like you're enjoying it, I'm throwing you out of the van while it's still moving."

Espo clapped his shoulder in reassurance. "Don't worry, bro. Beckett is like a sister to me. It'll be weird if I tried to cop a feel."

Usually, stuff like this wouldn't have bothered him. He trusted Esposito but something about this case had him on the edge. He had been nervous ever since Gates gave them the go ahead with the operation. Castle couldn't quite put a finger as to what bothered him so much. He had been through worse, seen worse. However, lately, any case that hit closer to home had Castle jumpy. As if his whole body was on edge, expecting the worst case scenario.

Castle caught the tech guy, who was sitting beside Kate and Esposito, gawking. Now Castle wished he had the same talk he had with Espo with him. Even though he was facing the bank of monitors in front of him, his gaze kept slanting towards Kate in that black dress. Now and then, Castle could see his beady little eyes roaming Kate's bare thigh as she hitched her dress up a little higher for Esposito to gain better excess.

Castle balled his fingers into a tight fist, doing everything he can to stop himself from punching the tech guy right in the face. Castle cleared his throat. Loudly. Kate and Esposito glanced at him. The tech guy snapped his eyes to one of the monitors, paling slightly. Good. "All set?" he asked the two detectives, he let his gaze linger on the tech guy a little longer before turning it to Kate and Esposito.

"Almost," Esposito said as he climbed to his feet. After a few more twists and turns and maneuvering of the dress, Kate was all done. She spoke low into the mic that was nestle between her breasts. The tech guy played on his keyboard for a moment before giving her gave a thumbs up. Esposito handed her an earpiece and a small handgun before turning his attention to the tech guy.

Kate turned to Castle and smiled. "You ok?" she asked softly as she took a step closer to him.

Castle looked up and returned her smile, saying, "Couldn't be better." Kate raised her eyebrows in skepticism but Castle didn't bother to reassure her.

"Can you help me with this?" she asked, handing him the gun. The small weapon was in its hostler that had Velcro straps on it.

"Sure," Castle mumbled as she parted the slit of her dress wider and presented him her thigh. Castle leaned forward while she placed her hand on his shoulder, bracing herself against him. He took his time strapping the gun.

"You know that this is all your fault right?"

Oh he knew alright. Him and his bright ideas. He should have just shut his mouth and let her lead the investigation like she has always done. But no. He had to open his big mouth and complain that sitting in the car, watching the suspect go in and out of his home, was boring. This case needed more action, he said.

"And what kind of action would that be, Castle?" Kate asked as she watched David Robins climbed out of his black Audi and made his way towards his brownstone home.

"We are not going to find anything about this guy by just sitting in the car and watching him. For the past three days, we only know that he changes his car as often as I change my underwear and he comes home with a different girl every night – all of them brunettes."

"What's you point?" Kate asked, glancing at him warily.

"My point is, there's nothing we can do with that information."

"Yeah, well, unless you have a better idea, this is the only thing we can do."

Excitement ran through Castle as an idea popped into his head. "Oh I do have a better idea."

"Yeah, I know," Castle replied as he tightened one of the Velcro straps. He let his fingers brushed lightly against her skin as he moved to the next strap.

Kate shifted, her hand travelling up his shoulder to the crook of his neck. Her thumb grazed his skin, brushing through the fine hairs there. "So don't be all pouty."

Castle lifted his eyes to Kate's. Though it was dark in the van, Castle caught her smirk. "I'm not being pouty," he said, scowling.

"No, that's an understatement. You look like you want to rip Steve's throat out."

"Who the hell is Steve?" Kate nodded to the tech guy behind her. Castle rolled his eyes. "Right. Well, Steve was leering. At you."

Kate laughed. "He's harmless." The strapping was done but he didn't remove his hands. He let them slid further up her thigh and felt goose bumps raise beneath his touch.

Kate's grip tightened against his pulse point as her eyes fluttered. "Castle," she breathed. Though her tone carried a warning, her eyes betrayed her true emotion.

Ignoring her, he asked, "Are you sure about this?"

Kate took a step back, their hands falling away from each other. "I'll be fine."

Castle stood, his gaze holding hers steady. "Ok, but if anything goes wrong…"

She smoothed out her dress as she replied, "I'll shout out for my knight in shining armor. And for you."

"That's all I ask."

"We're good to go," Esposito announced.

"I'll be fine," Kate repeated, softly. She tugged on the lapel of his coat, smiling, before turning away. He watched as Esposito helped her out of the van. Castle took a seat beside the tech guy – Steve was it? – and watched Kate through one of the monitors. They didn't have a camera on her so the only visual Castle had was of Kate walking from the van to the Robbins' brownstone where she was meeting him for dinner. As soon as she stepped inside, they would only have audio surveillance and nothing else.

"Mic test," came Kate's voice through the headphones Castle had on.

"We read you loud and clear, Detective Beckett," Steve said through his microphone.

Castle felt a hand land on his shoulder, making him jump slightly. Esposito towered behind him. "You're in my seat, bro."

"Oh right." Castle pulled off the headphones and vacated the chair.

"Any visual?" He heard Kate say, voice tiny and muffled now that he didn't have the headphones on.

"Nothing out of the ordinary," Steve replied as Esposito took his seat beside him. Castle dragged a chair from the back of the van and sat between the two cops. He placed both elbows on the back of their chairs and leaned forward, not wanting to miss a word.

Esposito glanced at him for a brief moment before speaking into his mic. "Ryan?"

"All clear," came Ryan's voice from Espo's headphone. The detective was considerate enough to have the headphone hang around his neck so Castle could hear everything that was going on. Ryan was sitting in his car at the end of the street, making sure they weren't going to run into any surprises. They still weren't sure whether Robbins worked alone or with a partner.

"Alright, boys. I'm going in," Kate said. A moment later, Castle saw her knocking on the door of the brownstone through one of the monitors. Within seconds, the door open and he caught a glimpse of Robbins before Kate was ushered in. The door closed behind her.

Castle glanced at Esposito expectantly. When he didn't say a word, Castle asked, "That's it?"

Esposito raised an eyebrow but didn't look at him, his attention on the monitors. "What's it?"

"You're not going to say – I don't know – 'the Eagle's in the nest' or something fancy like that?"

This time Esposito did look at him, confusion written all over his face. "What? Why would I say that?"

"That's what all the cops in the movies say."

Esposito shook his head, turning his attention back to the screens in front of him. "No, cops in real life don't say that. Besides, why would an eagle be in a nest?"

Caste shrugged. "It just sounded fancy." Even though the detective had his back to Castle, he could practically see the eye roll. He heard the tech guy snicker beside him and Castle shot him a dark look, quietening him. The three of them fell silent as they listened to Kate and Robbins have their date. Which basically meant that Castle had to listen to Robbins flirt openly with Kate. His Kate. Oh and it didn't end there. He had to listen to her flirt back.

He didn't know which was worse.

Their dinner went on for about an hour. They talked about his work – he was a college TA who recently came about a large sum of money when his father died nine months ago – and they talked about her work – she was an aspiring artist. They mused about how funny it was that they ran into each other in a gallery opening, especially when Robbins wasn't much of an art fan. He asked whether their meeting was fate. Castle snorted.

It wasn't fate. It was timing. Robbins was known to troll art gallery openings for his victims. He had a soft spot for artsy brunettes. He would invite the girl for coffee first. Then for dinner. On their third date, he would invite her to his place for dinner where he'll rape and murder them all in the same night.

In the past week alone, he left three bodies for the NYPD to find but they could never find enough evidence to convict him. He always seemed to have an alibi – which made them think he wasn't working alone – and not enough evidence for these killings. They needed more and they believe that they were able to find more evidence in his home.

After the first date with Kate, Castle had decided that he didn't like this Robbins guy – not that he was likable in the first place. Since their first two dates were in public places, they were able to watch Robbins and by watching him interact with Kate, Castle knew that Robbins was the scum of the universe.

Castle didn't like the way Robbins touched her or let his eyes roamed her body. He didn't like how close he sat with her or how he always seem to have a hand on her chair as if to keep her rooted in place. He didn't like the questions Robbins asked and how Robbins would dodge questions about himself.

And then there were the little things – the clothes he wore, the cars he drove, the company he kept. Robbins reminded Castle a bit of himself – back when he was the playboy partyer. He knew how guys like that work.

Now that Kate and Robbins were in the privacy of his own home, they didn't have the luxury of video surveillance and had to just rely on audio. And because of that, Castle had been on edge since the night began. He knew what Robbins was capable of and he hated not being able to see what Robbins is doing behind closed doors. What will he do now that he wasn't in public?

"So you ready for dessert?" Robbins asked. There was a tone in his voice that told Castle that Robbins wasn't just talking about cake.

"Sure. But first, can I use your bathroom? I would like to freshen up before we continue the night," came Kate's reply.

Castle could practically hear the grin in Robbins voice as he said, "Of course. It's just down the hall – second door on your right."

There was the sound of movement and a minute later, Kate's voice came spilling through the headphones. "I found his room."

"Roger that," Esposito replied.

"Everything seems to be standard here – nothing out of the ordinary…walk-in closet…dresser by the bed…bathroom."

Castle couldn't help but think about what Robbins was up to while Kate was rummaging around his bedroom. Was he sharping his knives? Finding that perfect rope? Drugging her wine? Adding poison to her dessert?

Castle gritted his teeth. Why did he have to torture himself like this?

"Everything seems to be ok. Nothing out of the ordinary."

"Get out of there befo –"

Espo glared at him over his shoulder and Castle immediately shut his trap. "Dude!"

"Sorry," he mumbled.

"Hey, everything alright?"

Castle froze in his seat when the voice that came through didn't belong to Kate.

"Oh I'm fine. I must have gotten turned around."

"Really? If I didn't know better, I thought you were just very curious about the bedroom." Kate laughed and Robbins chuckled with her. "How about I give you a tour of the place? That way you won't get caught snooping?"

Esposito and Castle exchanged worried looks. It was difficult to tell whether Robbins was joking or not.

"You know that sounds great but it's late. Maybe we can do the tour another time. I should get going anyway," Kate was saying.

"So soon?"

"Yeah, early start tomorrow but I had a great time. Dinner was great. You were great."

Robbins chuckled and when he spoke, his voice was louder, nearer. "How about we skip the whole next time and get right down to it?"

Kate laughed but Castle didn't miss the undercurrent of nervousness in the sound. "It's late. Maybe some other time," she repeated, this time a little forcefully.

"Come on, Katherine, just stay a little while longer. We still have more dessert to get through."

"We'll save that for later," Kate whispered teasingly. It was obvious that she was trying to distract Robbins. Castle hoped it wasn't all that obvious to Robbins himself.

Nothing was said after that. The only sound Castle could hear was the rustle of fabric against fabric and his mind started conjuring images that he wished it didn't. "Something's wrong," he whispered to Esposito.

"Hang tight, bro," Esposito said as he fiddled with a knob on the dash in front of him – probably turning the volume up. Was Esposito trying to kill him? The last thing he wanted to hear was the sound of clothes ripping from Kate's body at a higher volume.

"No, I mean it. Get her out of there. I don't like the sound of this."

Esposito shot him a look over his shoulder. "Everything's fine. She got this under control, if she didn't she would give us the signal."

"Maybe she's trying to fine the weapon on the guy's body," the tech guy cracked, grinning as he looked at Esposito. "You know, a full body search."

Esposito's expression darkened as he caught the tech guy's gaze. "Shut your mouth."

Castle clapped the tech guy on the shoulder and gave it a hard squeeze. "Listen – Steve is it? – why don't you try to find a way to see if we can hear more?"

Steve's eyes went to Castle's hand before flicking to his face. "What? This is all we have." Castle's grip tightened and a smiled almost escaped onto his lips when he saw a flash of pain in the other man's eyes. He looked over to the other detective beside him for help.

"Go," Esposito commanded. Without a second thought, Steve scrambled from his seat and went to the other side of the van. He started fiddling with a bunch of knobs there. Castle took the vacated seat and slipped the headphones on. "Sorry about that," Espo said in a low voice. "He doesn't know."

Castle grunted his response as he focused his attention on the audio.

"…Katherine …"

"Did you hear that?" Castle asked, alarmed. Was that a moan?

"Yeah but –"

Castle shushed him. His brow furrowed in concentration as his ears strained to hear more.

"Are you wearing a wire?!"

The voice exploded in Castle's ear, making him jump.

"Shit," Espo cursed beside him. "Ryan."

"On it," came Ryan's reply.

"David please…"

"What the hell is going on?"

"I –"

There was a noise that sounded like a smack or a slap. Castle's eyes widened. "What going on?" he asked, the tremble in his voice unmistakable.

"Beckett, can you read me?" Esposito said into his mic.

There was silence.

Castle's heart thudded in his chest, almost bursting through. This is not good. His hands flew to the headphones, pressing the ear buds closer to his ear – as if that action alone will help him to hear something. Anything.

"Fuck."

Ripping the headphones off, Castle said, "I'm going in."

"Castle, no!"

But Esposito was too slow. Castle was already clambering out of the van, his shoulder bursting through the doors and throwing himself out of the vehicle. His body was on high alert, eyes focused on the front door of the brownstone. He heard Esposito shouting for him but his brain barely registered it. He needed to get to Kate.

Thankfully the front door was unlocked. He barged through and raced up the stairs that led to the apartment. "Kate!" he yelled as he found himself in the living room. "Kate!" He heard a sound, a cry. Of surprise? Of pain? He wasn't sure. He sprinted towards the general direction of the sound, eyes scanning the area, searching desperately. His chest tightened as every minute passed. He felt like he was running out of time.

Leading by instinct, he headed towards the door that wasn't fully closed. A silver of light leaking through the crack between the door and its frame. He slammed a palm against the heavy wooden door. It crashed against the wall, startling its occupants. Robbins was pressed up against Kate, who was pressed against a wall next to the bed. One of his hands had a tight grip on her forearm while the other was sliding up her thigh, getting way to close to her hidden gun. He whipped his head when he heard the door bang open, surprise and anger mixing on his face. Kate looked shock.

The room disappeared.

Rage poured into his blood, filling his veins.

All he saw was David Robbins, the rapist, the murderer.

And his hands.

On Kate.

"Who the fuck –"

Robbins didn't get a chance to finish his sentence as Castle devoured the distance between them, grabbed the man and hauled him off his girlfriend.

"Castle!"

Castle shoved Robbins on the floor and climbed on top of him. Castle's body was humming with energy, with adrenaline. He was so full with anger that he felt like exploding. He needed to let it out somehow or he'll erupt and it would be so difficult to pick up the pieces.

Robbins' eyes widened in terror when he realized what was happening. Gripping the collar of his shirt with one hand, Castle started pummeling.

He heard a scream.

A groan.

Skin broke.

Bones cracked.

Blood poured.

Colours splashed across his vision. Red. White. More red.

He heard footsteps pounding towards him. He heard someone screaming his name again and again and again. He felt hands on him but he didn't stop. He kept swinging. He kept punching. Castle was focused. He was determined. All he saw was Robbins breaking beneath him.

Robbins the rapist, the murderer.

His hands on Kate.

"Castle! Stop! Please!"

Something in Kate's voice snapped him back to reality. He faltered. He felt hands on him again and this time the person was able to pull him away. His legs lashed out as his hands lost its hold on Robbins, hoping for some kind of contact. The hands on him tightened and he felt himself being dragged away. "You fucking touch her again and I will kill you!" Castle roared at Robbins. There was no response from Robbins. He seemed to be unconscious. Or maybe dead.

Castle's breath came out heavy and ragged as he struggled against the firm hold. There was still so much anger to burn. "You got to relax, bro," came Esposito's voice next to his ear. It was then when Castle realized that it was him who was dragging him away from Robbins, away from the room. "If you don't, I'm gonna have to cuff you."

"Take him outside, Espo," Kate commanded. The strong tone in her voice didn't fail to mask the fear that was underneath. Castle lifted his eyes to hers. Whatever anger that was in him disappeared when he caught the look on her face. Something heavy settled on his heart. He went numb.

She looked scared.

Of him.

"Take a walk, Castle," Esposito commanded as soon as they were outside. "Cool off."

Castle shrugged off his hands, ignoring his order. "Where's Beckett?"

"Castle –"

"Is she ok?" he asked, voice loud and high.

"Dude, she's fine," Esposito replied curtly. "She'll be out as soon as she done cleaning up your mess. Just…wait here. And don't do anything stupid." Without another word or a glance at him, Esposito jogged back into the apartment.

Castle started pacing. He shoved his hands into his pockets and he winced as his raw knuckles scrap against the rough material. He should get them checked out but he didn't care. The pain thrumming through his hand was a welcome distraction. He let the cold New York night cool off his hot skin as he paced in front of the brownstone anxiously. He felt like he was running a fever. His breath puffed out in front of him as he exhaled, trying to calm his thumping heart.

In the distant he heard sirens and moments later, the street was awash with blue and red lights. An ambulance pulled up beside him and two paramedics jumped out with a gurney. Castle decided it was time to put some distance between him and the brownstone. He didn't want to see the paramedics loading Robbins into a body bag.

"Castle!" He turned at her voice and saw Kate stalking towards him. Anger masked her features but Castle could see the hint of disappointment in the furrows on her brow. "What the hell, Castle?" she asked as soon as she was within earshot. Her voice was loud, catching the attention of a couple of uniformed police officers who had just arrived at the scene.

"Are you alright?" he asked, ignoring her question. "Did he hurt you?"

"Don't," she spat. "Don't give me that. This isn't about me."

Castle sighed as he raked a hand through his hair. "Look, I –"

"Did you know what you just did?" she asked. "Did you know how badly you screwed up in there?"

"Is he ok? Is he dead?"

Kate's eyes widened at his question. "Were you trying to kill him?"

"I –"

"Detective Beckett!"

Kate whirled around. Castle lifted his gaze. Gates stood a few feet behind them, hands on her hips, looking every inch the Iron Gates that she was. Castle didn't want to know how she got wind of the situation so quickly. She must have had an ear on them the whole time.

Shit.

"Let me go and explain to Gates," Castle started saying. Kate turned to face him and the rest of his words died in his mouth.

"You're done here, Castle. Go home and we'll talk about this later," she said. She turned on her heels and made her way towards Gates.

"Kate…!"

"Go home, Rick!" she threw at him without a backward glance.

Chapter 2 up soon. Thanks for reading. You're awesome.