*Yeah, yeah, I know, I'm a horrible person for getting sidetracked from my main fics again, but I'll get back to then, I promise. Oh, also, I know absolutely zip about forensics or just... science in general, so if anything in here looks completely made up or misused... that's probably because it is. Sorry. Serves me right for trying to write a Sci-Fi fic I suppose, but I really felt like doing a Castle/Doctor Who crossover. Nor do I know anything about life in New York City, so most of that's made up as well. So... enjoy? Oh, and this is the Eleventh Doctor by the way, sometime after The God Complex but before Angels Take Manhattan. As for Castle, it takes place at some time in Season Five after Alexis goes off to college, but the interaction between them is a bit like earlier series, sorry about that. If you like you can ignore them talking about Alexis being at college and pretend its in seasons two or three or something.*

Sometimes Castle believed the wild theories he came up with, and sometimes he just said them because he wished they were true. Life was more exciting when you let yourself fantasize about CIA conspiracies and psychics. And besides, his theories drove Beckett up the wall, which was incentive in itself. But he never, ever could have seen coming the one time he turned out to be right.

"Ah, Detective Beckett."

The brunette detective turned to face him as he extended her coffee out. She accepted the cup with a quick smile, which Castle warmly returned. "You took your time," she observed drily.

Castle shrugged good-naturedly. "Alexis popped by, I got held up." He let his gaze sweep over the crime scene. They were outside a parking garage, with yellow tape cutting off most of the area and the familiar swarm of cops and CSU hurrying about.

Beckett lifted the tape to let Castle through. "Isn't she doing that kind of often?" she asked curiously.

Castle's face fell. "What 's wrong with my daughter coming home?" he asked in an affronted tone.

Beckett rolled her eyes. "She's in college, Castle," she pointed out as they approached the crime scene. "I'm just surprised she's going back and forth so often. But I guess it does make sense."

Castle puffed up a bit, his pride restored. "That's what I thought."

"After all, there's free laundry at your place."

His face fell as quickly as before. He threw his girlfriend an injured look, muttering darkly.

Beckett tried and failed to hide a smirk, then turned to the CSU team gathered around the body.

"Alright Lanie, what have we got?"

The medical examiner was crouched over the body of a man. He was laid out on his back, his head resting to one side and his arms and legs thrown about awkwardly. Probably from landing or being pushed to the ground, Castle reasoned. He was young, probably late twenties early thirties, with curly red hair and quite a few freckles on his narrow face. With the glasses and the tie, he had a fairly reserved bookworm look to him.

"As far as I can tell, he's been here somewhere between eight to twelve hours, though I'll be able to narrow that down once I get him to the lab. The real interesting part," Lanie said with more interest, "is the laceration on his throat, which I'm calling as cause of death. Take a look." She pulled the man's shirt collar out of the way so they could better see the wound.

A long, horizontal cut ran across the victim's throat, with blood spattered on the shirt below. More interestingly, the skin around the cut was charred and black, fading out slowly to an angry red with muted blisters. Beckett's eyebrow furrowed. "Are those burn marks?" she asked incredulously.

Lanie nodded. "Looks like whatever cut down your victim was hot enough to burn the flesh."

Beckett's eyebrows flew up in surprise. "That badly?" Her gaze flew over the wound again, reassessing.

Castle's eyes widened. "A blade that was sharp enough to cut the flesh but hot enough to burn it?" At his excited tone, Lanie and Beckett turned to look at him with varying levels of interest and exasperation. Castle looked gleefully at his audience, making sure he had their attention, before announcing, "Isn't it obvious? He was killed with a lightsaber!"

The little attention he'd been able to command was lost immediately. Beckett rolled her eyes and turned her attention back to Lanie. "Got anything else?" she asked, pointedly ignoring her partner.

As Lanie shook her head, Ryan approached, notebook in hand. "His wallet identifies him as a Timothy Karrow, 35 years old, address 55 West 76th street, no keys on the body. He was found here this morning by some kids who work at the nearby restraunt. They got here around 6:30 this morning, found the body, called the cops. Said they've never seen him before." He gestured over to where a group of scared-looking teenagers were being questioned by another cop.

Beckett nodded briskly. "Alright, check around to see if he might have dropped his keys. If he was parked here, maybe he worked nearby. In the meantime we'll check and see if he was reported missing and contact his family. C'mon Castle." She got to her feet and started off for the car, leaving her partner to hurry after.

They were almost to the car when Beckett paused, eyes narrowed. Ahead of them a strange man was talking animatedly to one of the uniforms standing by the yellow tape. Beckett and Castle came up to stand beside the cop. "What's going on here?" Beckett asked calmly.

The cop seemed grateful for the interruption. "This guy here," he gestured irritably at the stranger, "says he's with CSU, but he's not showing me any ID."

Castle turned his attention to the stranger. He was wearing a brown tweed jacket with a pale undershirt and a red bow-tie, with brown hair that was swept to one side. Currently, he was rummaging through his pockets, looking very put out when all he was able to drag out was a pair of 3D glasses and a squished bag of jelly babies. He looked up sheepishly. "I did have qualifications," he said in a British accent, "I just seem to have misplaced it... erm, somewhere." He gave his pockets a last glare before straightening his bow-tie nervously. Castle had to bite back a laugh at the man's eccentric appearance, but Beckett was evaluating the man carefully, her eyes narrowing at what she found.

"What's your name?" She asked abruptly.

The man looked at her in surprise and a hint of irritation. "John Smith," he offered dismissively. He drew a silver metal thing rather like a pen from his coat pocket and held it out toward the crime scene. Something green lit up on the end and it let out a whirring sound. As the end snapped back into place, John held up the device, studying it closely. His face fell. "Ooo, that's not good. That's quite a bit not good."

Castle perked up at the sight of the weird pen-thing. "What's that?" He asked eagerly.

John cast the cop, who still wasn't letting him through, a quick irritated glance. "This," he said, indicating the pen-thing, "is my tool... thingy. It's doing something complicated and brilliant that's telling me that something's really very wrong with the body in there. Now, if you could just let me in..." He tried to slip past the cop, but his way was firmly blocked. With a childish pout, he backed away.

Beckett crossed her arms, eyes still carefully evaluating the man. "Mr. Smith, if I ask anyone from CSU here, are they going to recognize you?" The man didn't respond, throwing Beckett a somewhat sullen glance. "I can call up the precinct right now and find out if you're lying, so how about you just save me a lot of time and work?"

John glared at her with all the sullenness of a child. Finally, he admitted, "Alright, maybe that was just a clever lie, but I'm here to help, so I really don't see..."

He was cut off as Beckett strode up to him and pulled out her handcuffs. "Alright, turn around and hands behind your back."

John gaped at her incredulously. "Really? You're arresting me? What for?"

Clearly having gotten impatient with his disbelief, Beckett grabbed John by the shoulder and forced him to turn his back to her. "For attempting to break into a crime scene, obstruction of justice, false identification... you name it." She snapped the cuffs on the man and directed him towards their car. John ducked his head and allowed himself to be shut into the backseat, glancing around the car with interest. He grinned brightly as Beckett started up the car and backed them out of the lot. "Huh, look at me arrested, never been arrested before. Well, that's not true, actually very not true, but look at all this then! You've got the handcuffs, the badge, even the car! I love a good police car."

Beckett just rolled her eyes and ignored him, but Castle turned to face the man. "So, what's your real name?" His partner glanced at him, surprised. "Oh, come on. John Smith? That's the ultimate go-to fake name. There's no way that's his real name."

Far from trying to deny it, the man just stuck out a hand, which Castle shook bemusedly. "Quite right. I'm the Doctor."

Castle raised an eyebrow. "Doctor who?"

"Just the Doctor," the man insisted cheerfully. He leaned forward so he was between Castle and Beckett. "Look at this!" He waved the sonic screwdriver over the console of the car, grinning as the device let out a brisk whirr. "It's a radio! Oh and look, that switches on the siren things!" He sniffed. "Of course, it's got nothing on my TARDIS, but all the same, you've gotta love a flashing light."

He reached out to poke something on the console, but Beckett sent him a glare she usually reserved for Castle. "Sit down and keep quiet," she snapped.

Looking a bit put out, the Doctor leaned back into his seat. Beckett turned her glare to Castle, as though blaming him for their suspect's annoying behavior. He just grinned back.

A few minutes passed in silence. Then the Doctor took out the metal tool thing again. It let out that whirring noise from before as he studied it with a furrowed brow. Castle glanced at Beckett's irritated expression, but in the end his curiosity won out. "What exactly is that?"

The Doctor continued to study it as he answered, "This is my sonic screwdriver." He looked up at Castle's confused expression. "It's a screwdriver... only it's sonic..." He looked down at the device again. "More importantly, it's reading low levels of tyron energy. That's defenitely not good. Tyron energy comes off of an advanced form of energy bomb. A bomb like that could punch a hole in this planet the size of Belgium."

Beckett had been looking at the road ahead with exasperation at the Doctor's babbling, but at the word "bomb" she stiffened. "You're saying you think there was a bomb nearby?"

The Doctor shook his head. "No, not nearby, the readings are too low for that, but someone in the parking garage had had recent contact with one." He adjusted his bow-tie, seemingly without even noticing what he was doing. "The problem is, this kind of technology's not supposed to exist for another few thousand years, so whoever was there is from the future."

Beckett's interest dissipated immediately. She threw Castle a look that plainly said This guy is nuts, which turned into a glare as she took in Castle's enraptured expression as he listened to the Doctor's words. "Listen, we're going to need your actual name."

The man blinked in surprise. "I told you, I'm the Doctor."

"Yeah, I got that part," Beckett said, rolling her eyes, "but Doctor who?"

"It's just the Doctor. Why is everyone always surprised? I like 'the Doctor.'"

The detective gave a quick glare over her shoulder. "What, it says 'The Doctor' on your birth certificate?"

The Doctor sighed. "There is no birth certificate," he explained patiently. "You're not going to find me in any database or catalogue you have, and I'm not a citizen of any country."

Beckett raised an eyebrow, a disbelieving smirk playing on her lips. "So what, you don't exist or something?" she asked.

"Oh, I exist," the Doctor assured her cheerily, "I just don't live on Earth."

Castle leaned forward, child-like excitement bubbling up inside him. "So where do you live then?"

The Doctor grinned. "In my TARDIS."

Before Castle could ask what a TARDIS was, Beckett sent him a silent glare clearly telling him to zip it, then turned back to the Doctor. "Alright, we'll need to get your fingerprints so we can run them through our files."

The Doctor grinned smugly. "If you think it'll help."


A few hours later, Castle came back to the desk from grabbing coffee to find Beckett hanging up the phone. She looked up and gave a grateful smile as she accepted the coffee. "That was Ryan. They found Karrow's keys swept under a car near where the body was found, and his car wasn't too far away."

She turned back to her computer screen. "In the meantime, I found out Karrow's got a wife who died of a stroke a few years back, three sons, and a daughter. After we interview Smith I want to run down to talk to them. I also found out Karrow works for a bookstore called The Secret Garden off of 51st. Ryan's checking it out now."

Castle sat comfortably in his chair across from Beckett's desk. "So, if this Doctor is involved with Karrow's murder, why would he try to break back into the crime scene?"

"Maybe there was evidence on the body that he wanted to remove?"

"Yeah, but Lanie said the body had been there for over eight hours." Castle pointed out. "He could't have removed any evidence before then?"

Beckett shot Castle a look that was half irritation, half amusement. "You just don't want him to be the killer because he's got a cool toy and he talks about time travel."

Castle abandoned pretense and let himself grin eagerly. "Oh, come on. First the body got sliced by a lightsaber, then the guy starts finding that Tyron energy thing, and he's dressed like some college professor. It's so obvious! He's a time traveller!"

His partner raised an eyebrow. "Right, so he came from the future to help solve a murder." Sarcasm dripped from her tone.

The writer nodded. "He said the killer had to come from the future, so the Doctor must have followed him here by tracing that turin energy. It all makes sense!"

"Yeah, except for the part where there's no such thing as tyron energy," Beckett pointed out drily. "Face it Castle, he's just a delusional criminal who won't even give us a proper name."

"And what makes you say he's a criminal?" Castle asked with a raised eyebrow.

"He said in the car that he'd been arrested before, remember? Once his fingerprints are taken I'm going to run them through our files to see if I can get a match." She stood. "Smith's waiting in the interrogation room. Let's go see what he says."

They were interrupted as a woman's voice called out, "Detective Beckett?" They both turned to see a woman with pale, frizzy hair approaching their desk. She was older than Beckett, somewhere in her forties, but she walked without the brisk professionalism of the other cops. No, Castle decided, it was more of a swagger built from confidence, fit more for a lawyer than a cop. She held up a wallet with a police badge for Beckett to see. "Detective Melody S. Cage, I'm here about the suspect you brought in earlier, a John Smith, might identify himself as a doctor?"

Beckett raised an eyebrow in surprise. "You're interested in the Karrow murder?" There was a slight edge in her voice. "That's my case. Do you have something about our suspect we need to know?"

The woman flashed an unfriendly smile. "Maybe. Let's just say I've run into him before." She smirked, as if there was a joke here no one else was understanding. "I'll need to talk to him alone. All right?" Not bothering to wait for an answer she turned and strode off towards the interrogation room.

Beckett gaped at the retreating Detective. For a moment, both she and Castle were lost for words. Did that actually just happen? Castle wondered gleefully. Finally she pushed up out of her chair and marched after the woman. "Detective Cage, I don't know how they run things at your precinct, but this is my case." She moved to block Cage's way, her expression steely. "You can watch if you want, but I'm running this my way."

Cage appraised the woman in front of her with a grin that almost seemed dangerous. She seemed to like what she saw, for her grin grew wider. "Oh, feisty this one," she commented in a voice like a purr. When Beckett didn't react, her tone grew more professional. "Listen honey, I've got an all-access pass." She flashed her badge again, but something on it must have changed. When Beckett read it again, she clenched her jaw unhappily, but moved out of her way. Cage chuckled. "Much obliged," she said with a little mock salute. She started to open the door, then turned back to Beckett. "Oh, by the way, this one's a bit mental. Delusions and the like. Seems to think he's a time traveler and that I'm his wife. Poor dear. Like he'd have a chance with me. I'm just going to play along a bit, don't mind me." With that, she slipped into the room.

From his spot by the desk, Castle was fighting back a grin. Beckett looked positively livid that this woman had stolen her suspect. Oh, this is going to be fun! Anyone who could get under Beckett's skin was alright in his book. The detective and her partner headed into the observation room to watch through the one-way glass, not bothering to look at his obvious glee as she muttered, "Shut it, Castle."

Still trying not to grin, he replied, "Wasn't saying anything."

In the interrogation room the suspect hadn't looked up as the Detective entered the room. He let out a patient sigh, looking down at his hands. "Here we go. Listen, I don't know what leads you're following, but..." He trailed off when he finally lifted his head and saw who had entered the room. His eyes widened in both recognition and surprise.

Detective Cage ignored the surprised suspect and made her way to the table. If there had any professional bearing in her walk before, it was completely shed now. Cage was smirking brightly, her posture becoming more relaxed. She lowered herself into the chair across from him and leaned back with an air of complete ease, finally flashing him a smug grin. "Hello sweetie."

Alright, so I'm still not quite sure how many chapters this thing's going to have, but I have most of the plot planned out. If anything here is horribly inaccurate, please call me out on it. I'll keep working on my other fics too, don't worry.