I know I'm taking a major gamble by posting this. I've never written a crossover before, but always wanted to have a go at it, and I have to say, I had great fun writing this.

Disclaimer: I own neither 'Castle' nor 'The Mentalist' and nothing affiliated with either of them. If I did, I'd make sure they weren't on rival networks so a crossover might actually be a possibility and not just a figment of my imagination.

Shout-outs to Shani8 who actually gave me the idea to tackle a crossover in one of her reviews, and to my pal ScientificAnomaly who let me run the theory by her and told me what she thought.

Rated T for language, sexual references and so on.

This story is set in late Season 3 of both shows, but before the season finales (we haven't seen them yet here so I don't know what's happened.)

I hope you like it.

"I still don't understand why you agreed to this, Lisbon."

Patrick Jane was stretched out on the couch (the one he had purchased for her as a matter of fact) in his boss's office, watching her add her signature on yet another form.

"I didn't agree to it," she said irritably, reaching for the next one. "The decision came from way over my head. But unlike you, I have respect for my superiors and obey orders." She flicked her gaze up to meet his, sternly.

"I have respect for you," said Jane indignantly. "And I do so obey orders…sometimes."

"Right," she said, rolling her eyes. "You'll forgive me if I don't just take your word for it, given your track record."

"Meh," he said, waving a hand dismissively. "Stop living in the past Lisbon. Look forward, not backward, and let the future take care of itself."

"How very profound of you," she said, turning her attention back to her work. But of course, she only got about five seconds of peace before he spoke again.

"You're taking this a lot better than I thought you would. I was expecting you to be all moody today and needing me to cheer you up."

She scoffed. "What part of you hanging around in my office making irrelevant comments is supposed to cheer me up?"

"You know you always feel better when you get to yell at me about something," he said. "Just think, if I started behaving myself you'd have to be nice to me all day long."

"As opposed to being practically able to feel the stress induced ulcer you're giving me getting bigger and bigger, all day long," she shot back.

"So what time are we expecting our brothers-in-arms from the Big Apple?" asked Jane. Lisbon glanced at the clock on her desk.

"Plane lands in twenty minutes," she said. "They're going to check in to their hotel and then come here, so maybe an hour, hour and a half."

"They're going to be very disappointed if they come all this way and it turns out this isn't the guy they think it is," said Jane.

"Well the MO is very similar," said Lisbon reaching for their current case file. "If it's not this serial they've been chasing, it's a very thorough copycat. Either way, they can help us."

"Take over the whole case, more like," said Jane. "Just you watch, they'll burst in here all gung-ho and tell us to sign it over to them-"

"That's not going to happen," Lisbon said, cutting him off. "I can deal with a joint investigation, but I'm not going to be pushed off my own case." She blew out a sigh. "This was such a bad idea," she said. "This whole thing is nothing more than a jurisdictional pissing contest just waiting to happen."

Jane hid a smirk at these words. He'd known she was way more pissed off about this arrangement then she was letting on, and now finally, the cracks in Teresa Lisbon's poker face were beginning to show.

It had all started three days ago, on that horrible day when they had been called to not one, but three crime scenes, each victim a young woman with blonde hair and ligature marks on their necks, suggesting that they'd been strangled. It hadn't been necessary to have Jane's mental prowess to figure out that this was the work of a serial killer, and a little research had matched the style of killing and victimology to the handiwork of The Triple Killer or '3XK' last seen in New York earlier in the year.

In an effort to find out more about what exactly they were up against, Lisbon had reached out to the NYPD to see what information they could share about 3XK. She was put through to Captain Roy Montgomery of the Homicide division and as soon as he heard the reason for her call, he'd asked to speak to her superior. Before she knew it, Lisbon's simple fact finding mission turned into a bargain being struck between Montgomery and Agent Bertram that a team from the NYPD would fly out immediately to assist with the investigation.

Jane, for one, was actually quite looking forward to the arrival of their new 'teammates.' It was like he was about to take delivery of some brand-new toys. It meant new people to irritate, new secrets to uncover, new chances to point out uncomfortable truths that would make them squirm. And it would be fascinating to see how they integrated themselves in with the CBI team.

Lisbon he knew, was anticipating their coming with a lot more apprehension. He knew she was concerned about how they would all be able to work together and he also suspected that she was quite keen to make it clear to them that being in her jurisdiction, she was in charge. The control freak side of her he was sure would be in full effect until this was all over, and he looked forward to seeing how this went down with the NYPD detectives.

All in all, he thought the whole thing was going to be very interesting, and rather fun. Well, there was nothing fun about death and murder of course, but he suspected he was going to be treated to some excellent fireworks between the two teams as they tried to solve it.

And he wasn't going to do anything to inflame the situation, should one arise. He would simply sit back and let the events play out any way they would.

Well…he might be forced to intervene if it ever looked like the other team was getting the upper hand. He might enjoy drama, but if it came down to taking sides, his loyalty was to the CBI, or rather, to Lisbon.

They might disagree on many counts, and irritate each other more than anyone else on the planet, but he always hated to see her upset. She was good at hiding it, but he could always tell. He sometimes thought of himself as a studier of human character, and she was his absolute favourite subject.

"Jane?" she said, breaking into his train of thought.

"Yes dear?" he said, comfortably, chuckling as she narrowed her eyes at him.

"Don't do that," she said. "I want you to promise me that you're not going to poke and prod at them when they get here. This case must be very personal to them for them to be making all this effort, and the last thing I need is you stirring up trouble again."

"Oh relax," he said, stretching his arms over his head. "This whole thing will probably end up being just a big misunderstanding anyway."

"Until we know for sure, we have to give them the benefit of the doubt," she said, firmly. "Imagine if it was us in their shoes. If it was Red John, wouldn't you expect to be extended the same courtesy?"

He stiffened at the mention of the serial killer's name. "It'd never happen," he said. "Red John will never kill anywhere but California. He has to make sure I know he's close by so he can taunt me every time he strikes and we don't catch him."

"But what if he did?" Lisbon persisted. "What if we got a call tomorrow that someone in Texas had been cut up Red John style? What would you do?"

What would he do? He'd like to say that he'd know it couldn't really be Red John, and that he'd do nothing, but he knew that he'd want to see it for himself, hunt for clues, find any link, no matter how tenuous, that might finally allow him to track the bastard down.

"I'd be on a plane to Texas," he admitted. He'd fly clear across the country, hell, he'd fly to fricking Australia if there was even the slightest chance of getting more insight into the monster that had destroyed his whole life.

"We would be on a plane to Texas," Lisbon corrected him, gently. "You think I'd let you go on your own?"

He shook his head. He had no doubt that she'd go wherever he went. But, there or not, she wouldn't be able to stop him taking his final revenge. He didn't want to hurt her, but at the end of the day, he was going to kill Red John, with or without her support. He needed to. He owed it to his family.

"I know you wouldn't," he said. Their eyes met and they exchanged a smile. A warm sensation seemed to settle at the pit of his stomach. That was what made her so special. Nobody else could make him feel like that, well at least, nobody had since Angela.

And he had to admit, even Angela hadn't had eyes as hypnotic as Lisbon's.

"So, back to the original point," Lisbon said, breaking him out of his reverie. "You have to promise me, no funny business, no rattling their cage, no stupid stunts, no practical jokes, no magic tricks, no mind-reading, no nothing. Best behaviour. Got it?"

"Aww, Lisbon," he complained. "That's no fun."

"Promise me," she repeated.

Those eyes again. She shouldn't be allowed to use them against him like this. It wasn't fair. He couldn't say no to her with her looking at him like that.

Surreptitiously, he crossed his fingers behind his back.

"I promise," he said.

She smiled at him again. "Thank you. I know you probably think I'm being overbearing, but I doubt anyone else will be able to handle you. As far as consultants go, you're pretty much one of a kind."

"Referring of course to my intelligence and charm?"

"Referring to the fact that you're a pain in the ass!" she said sharply. "I'll bet the other team don't have an annoying consultant. Must be nice to not have someone shadowing you all the time."

How Beckett hated the new aviation laws that forbade the carrying of guns on planes. If hers were handy, she fancied she'd be pointing it at Castle right now, or maybe just beating him over the head with it.

Either one.

Across the aisle, she could see Ryan and Esposito trying to muffle their sniggers as Castle tugged on her sleeve like an overexcited toddler.

"What?" she snapped at him, a little louder then she'd intended, causing a passing stewardess to look over at her, eyebrows raised.

"There's no need to be so aggressive," said Castle, throwing up his hands in surrender. "I was just going to ask you if you wanted some candy."

He brandished a brightly coloured tube of candy at her.

"No thanks," she said, testily. "I'm trying to watch my sugar intake. And so should you," she added, snatching it away from him. "You're hyperactive enough, and it's getting on my nerves."

"Are you sure it's me that's irritating you?" he asked. "Or the lack of legroom in this coach class seat?"

"Will you give that a rest?" she said.

"I'm just saying, if you'd accepted my generous offer you could be reclining in a nice first class seat right about now with a glass of champagne in your hand in the peace and quiet, instead of here."

As if to emphasise his point, the baby right behind her promptly began to wail.

She bit her lip to keep from answering, pulled the in-flight magazine from its pocket, opened it at random and pretended to be absorbed in an article.

Beside her, she heard him chuckle, and the scratching of his pen as he resumed writing in his notebook, presumably creating more escapades for Nikki Heat.

She had been tempted when he'd offered to spring for first class seats for the two of them, she'd be lying if she said she wasn't. But this was a trip with her whole team and it didn't feel very teamlike to go and live it up in first class while making Ryan and Esposito languish in coach.

So being a good boss, she'd opted to ride with the boys, and of course after he'd heard her decision Castle had immediately chosen to slum it too, apparently there was no question of her being separated from her shadow, even for one plane flight.

And so here they all were, wedged in like sardines, on the way to Sacramento.

She'd been shocked when Captain Montgomery had called her into his office to tell her that 3XK had resurfaced, and in California of all places. She'd always had the idea he'd come back to New York. But perhaps this was just some other way for him screw with them, by making them chase him all over the country.

Well that was fine with her. Her team had some unfinished business to settle with the serial killer and if that meant they had to rack up some air miles to do it, then so be it. Unfortunately, his change of state meant that the case had fallen into the hands of the California Bureau of Investigation, and they had been unwilling to give it up completely. So Beckett had been forced to consent to sharing the case with them.

But she wasn't too worried. Surely the CBI was just as busy as they were and had just as many, if not more, open cases. She was quietly confident she would be able to get the agent in charge to let them take 3XK off their hands. All they needed was to have a talk, cop to cop, and then the CBI could get back to chasing Californian criminals, she and her team could finally take down 3XK, and everybody could go their separate ways.

There was a pause in the scribbling beside her, and she carefully glanced over to see Castle twirling it in his fingers, apparently lost in thought. She hadn't quite been able to gauge how he was feeling about all this; after she'd first told him, he'd gone very quiet for a while, but then before she knew it, he was crowing about the free trip to California, like they were simply going on vacation rather than something more like a witch hunt.

She'd tried suggesting that he stay in New York, assured him that it wasn't essential that he accompany them, and wouldn't Alexis miss him terribly while he was gone?

But of course, he wouldn't hear of missing out and swore to her that he had Alexis's full blessing to go. In fact, by the sounds of it, the teen had been a little too encouraging, and if it were anyone else, Beckett might have suspected she was planning to throw a party. But this was Alexis, and she probably was just eager to have an empty house to get some study done.

Sometimes she really couldn't believe Castle could have fathered a daughter as diligent and responsible as Alexis. He, the eternal child, and she, seventeen going on thirty. How had the traditional father/daughter roles been reversed so dramatically?

Of course, Alexis may not have been quite so quick to give her permission if her father had told her the real reason he was going. Castle had confided in Beckett that he had chosen not to tell Alexis about 3XK, he didn't want her to worry, he said, and he really felt that it was important for him to go.

Beckett understood his reasoning. After everything that had happened last time, including being kidnapped, she understood that he had a score to settle with 3XK, and she could hardly begrudge him for wanting to catch the guy. She had practically built her entire career around the same principle; finding her mother's killer. It would be hypocritical of her to forbid him to get the same closure she was so desperately seeking.

She also knew he felt a certain responsibility for the three new victims, even though it was in no way his fault. She'd tried to tell him that, but it had fallen on deaf ears, as she'd expected. She knew how that felt too.

Jane had finally managed to fall back asleep on Lisbon's couch, before he was rudely awakened by a knock on the door. He cracked an eye open to see Cho standing outside of it, and shot him an angry look, but for all the notice Cho took, he might as well not have bothered.

"Just got a call from the front desk," he said, addressing Lisbon. "They're here, and on their way up."

"OK, Cho. Thanks."

Cho nodded once, and left. Jane watched as Lisbon got out of her chair, smoothing her hair back and straightening her pendant.

"Stop it," he told her. "You look lovely."

She turned faintly pink. "Jane, that's sweet, but not what I was going for."

"I know, I know," he said shaking his head. "You're trying to look professional and intimidating. I get it."

"Is it working?" she asked nervously, in a rare moment of vulnerability.

"Just give them the old 'Lisbon glare.' That'll sort them out," he said, hoisting himself off the couch.

"The what?"

"You know, that look you sometimes give people, when you're really annoyed about something."

"You mean the one I'm always giving you?" she said, all trace of anxiety now gone.

"Exactly," he said, opening the door to the office for her. "Seven years working together and it still scares the hell out of me."

"If it's so scary, why is it still not enough to make you do what I say?"

He grinned at her. "I refuse to let my fear hold me back," he said.

They walked out into the bullpen to see four people, three men and a woman, exiting the elevator. Beside him, he saw Lisbon straighten up, holding her head high, as she walked forward to greet them.

He studied the group intently. The woman was flanked on either side by the three men, but they were all standing an inch or so back from her, suggesting she was the one in charge. Interesting. Lisbon was quite used to handling her male counterparts, but it wasn't often that she met another woman in a position of power like herself.

There seemed to be something different about the man who stood to the woman's immediate left. His suit was tailored, and easily looked twice as expensive as the others. His shoes were Italian leather, and he was looking around the bullpen with great interest. Jane guessed that this man was not a cop. So what was he?

Lisbon had reached them now and she extended her hand to the woman.

"Welcome to the California Bureau of Investigation," she said. "I'm Agent Teresa Lisbon."

Jane registered the use of her full title, and hid a smile. Typical Lisbon, trying to establish her authority right away.

The other woman shook her hand. "Detective Kate Beckett," she said. "NYPD."

She held Lisbon's gaze; clearly she was no stranger to control tactics, and it also helped that she towered over Lisbon in height, thought admittedly she was wearing stiletto heels.

"Nice to meet you, Detective Beckett," said Lisbon. "I'm sorry it had to be under these circumstances."

"Just 'Beckett' is fine," the other woman said, ignoring the rest of the statement. "This is my team. Kevin Ryan, Javier Esposito, and Richard Castle." She indicated each as she introduced them. Jane felt recognition register at the last name. Richard Castle. He was sure he knew that from somewhere.

Lisbon nodded briskly, and then turned away from them to her own team.

"This is Wayne Rigsby, Grace Van Pelt, Kimball Cho," she broke off as Jane came to stand beside her. "And this is Patrick Jane."

"Enchanteé," said Jane, and he couldn't resist shooting a grin at Beckett, who he realised now he was up close, was nothing short of stunning. Surely she could have easily been a model, a beautiful woman like that, but yet she'd chosen to become a cop. He made a mental note to find out why.

"It's a pleasure to meet you," he said.

Predictably, Lisbon rolled her eyes, but Detective Beckett was taken aback for only a moment before regaining her composure.

"Smooth," she commented. "But you're lucky you didn't try to kiss my hand, because I'd have broken your nose."

Her three team members snickered to themselves, and even Lisbon cracked a smile.

"I consider myself duly warned," said Jane.

"Good," said Beckett.

"Sorry about him," Lisbon said, as though he was not present. "He's just a consultant."

"Don't worry about it. I've got one too," she jerked a thumb in Castle's direction. "If yours is anything like mine, I know where you're coming from."

"Thanks a lot," piped up Castle, glaring at Beckett. "I am a human being you know."

"So you keep telling me," she retorted. "I'm still waiting for the day when you actually prove it."

Castle looked slightly hurt for a moment, but grinned back at her, which she eventually returned. It appeared that this was quite a normal occurrence for them, which was unusual. The only other relationship he knew like it was between himself and Lisbon.

"Well now everyone's met, should we get down to business?" Beckett enquired.

"Good idea. Let's go to my office and figure out where we start."

"Stay here," Beckett instructed her team. "Get to know the others a little." Ryan and Esposito both nodded, but Castle's face fell.

"Come on Beckett, we always do briefings together. We're partners."

"Not this time," she said firmly. "Stay."

"Jane that goes for you too," said Lisbon. "And if you eavesdrop under the door, or pick the lock then I'll-"

"Kick my ass," Jane supplied, casually. "Got it."

The two women walked into Lisbon's office and shut the door. He noticed Castle wistfully following his partner's progress across the room, and also the way he didn't immediately look away when the door closed. Now that was interesting too.

It was just as he'd hoped. Secrets and intrigue galore.

Jane smiled to himself as Grace, ever the social butterfly and peacemaker, got up from her chair and went over to Ryan and Esposito.

"Hi, I'm Grace," she said smiling shyly at them both. "It's nice to meet you." She shook hands with each of them in turn. "Come meet the others," she said and they followed her over to where Rigsby sat, and he too extended a hand to each of them.

Cho lifted his head from the book he was reading as they approached, said a quick 'Hey' and then returned to the book without further ado. The two detectives looked slightly affronted, until Van Pelt took it upon herself to explain.

"Don't mind Cho," she said. "He doesn't mean to be rude, he's just like that. Would you guys like a coffee?" she asked.

"Sure," said Esposito. "Anything will beat the crap we had on the plane, hey bro?" he said to Ryan.

"Too bad we couldn't bring Castle's espresso machine," his partner replied. "But there was no way we could pass that off as carry-on luggage."

Van Pelt smiled at them. "Break room's through here," she said. "Come with me."

Castle watched as Ryan and Esposito followed the pretty redhead out the door. He noticed Esposito throwing her appreciative looks and made a mental note to threaten him with Lanie later. Not that he ever would cheat on her, but still, it'd be fun to tease him about for a while.

"So, Richard Castle," the blonde guy, Patrick, with the curly hair was addressing him now. "The name rings a bell."

"I'm a novelist," he said.

"Ah," said Patrick nodding his head. "That explains it then."

"Explains what?"

"Well it's obvious to anyone that you're not a cop. The way you're dressed, even the way you carry yourself. So how did you end up working with Detective Beckett and her team?"

"I'm shadowing her as research for my books," he explained, for what felt like the millionth time. "I'm basing a character on her."

"Nikki Heat?" asked Jane.

"Yes! You've read Heat Wave?" Castle asked, thrilled at the prospect of meeting another fan, but Jane shook his head.

"No, I'm not much of a reader," he said. "Cho's read it though."

"Then how did you know Nikki was based on Beckett?"

"I've seen the cover of 'Heat Wave.' I can see the resemblance."

Castle grinned to himself, remembering the way Beckett had reacted when she'd seen the cover art for the first time, and how she'd hit the roof even worse when she'd seen the one for 'Naked Heat.'

"You know, you look familiar too," he said now, scrutinising the other man. "I think my mother used to watch your psychic show."

"There's no such things as psychics," said Jane, shortly.

"I'm sure it was your show," Castle persisted. "And I remember one time Mother and I were in Las Vegas for a play she was doing, and you were performing at the time. She used her only night off just to see you, and spent an extra hour on her hair and make-up, just in case you called her up on stage."

Jane hadn't cracked a smile during this anecdote. In fact, he looked an odd mixture of angry and sad.

"I haven't been doing that stuff for a long time," he said, politely, but with a finality that made it clear the subject was now closed.

Castle didn't press any more, but he was incredibly intrigued by this man. There was a story here; he was sure of it, and as a writer, he was deeply curious to find out how it went.

Ryan and Esposito walked back into the room, each carrying a cup of coffee, with Van Pelt, who was carrying two. She offered one to Castle and he accepted it, smiling at her.

"Not exactly café quality I'm afraid," she said. "Sorry."

"Is the other one for me?" piped up Rigsby, from across the room.

'Do I look like a barista, Wayne?" she snapped at him. "You know where it is, get it yourself."

He scowled at her, but then turned to Ryan who was closest.

"Did you say you guys have an espresso machine?"

"Yep. Castle here bought it after he started working with us. One of the perks of having him tag along."

"We should get one of those," Rigsby said, looking to Jane now. "You're always spending tons of money on stuff for Lisbon, why not use some on something we can all enjoy?"

Jane didn't answer right away; Castle fancied he was lost in thought.

"Jane? Jane!"

He snapped out of it.

"What? No, I'm not doing it, Rigsby. Coffee cart's just downstairs, that should be good enough."

"Besides you drink too much of it as it is," remarked Cho, not even looking up from his book as he turned the page.

"And so does Lisbon," added Jane. "Which reminds me, it's almost twelve. She'll be storming out here in a minute looking for her lunchtime caffeine hit. I should go take her one, otherwise we'll all be paying the price."

Castle saw him smile a little as he said this. Perhaps Patrick Jane had a little thing for his boss? The plot thickened. And now he came to think of it, Beckett would probably be hanging out for a coffee too right about now.

Before either of them could move however, everyone's attention was caught by raised voices coming from Lisbon's office.

"Agent Lisbon," he heard his partner's irritated voice say. "This case belongs to us, as does the arrest when we catch him. 3XK kidnapped two of my team members, one of whom was injured, and then he slipped through our fingers. I can't let that stand."

"I understand your motivation," Lisbon immediately shot back, "but the fact remains that this is now in CBI jurisdiction. You were invited here as a gesture of good faith, but if necessary, that invitation can and will be revoked."

"Is that some kind of threat?"

"Not at all. I have no objection to your team's remaining here in an advisory role, at least until we can establish whether this really is 3XK, but I do want to make it clear that as far as making decisions on how to proceed goes, I'm calling the shots."

There was silence for a minute. Castle found he was slightly impressed at the CBI agent. Never before had he seen somebody go toe-to-toe with Beckett and hold their own so well.

He was forced to revise his first impression of Agent Lisbon. At first he'd been surprised to find out she was the head of the team, being so petite and frankly looking as small as a child next to her agents (Rigsby in particular) but he could see quite clearly now that what the woman lacked in height, she made up for in tenacity.

Just like Beckett. Oh, there were going to be fireworks before this thing was over

"This is my case," he heard Beckett say, in that icy calm voice that always meant trouble and had him running for cover. "My team. My suspect, who struck in my backyard. Throw me off the case if you want to, but I'm not backing down from this. This bastard hurt people I care about, and I will take him out, with or without your help."

People she cared about. Castle felt an internal thrill at these words. Beckett cared about him. Of course, he'd always known that, but it was nice to hear her say it all the same. And by God, did he care about her too.

Always.

"He may have started off in your backyard," Lisbon said, calmly, but with a definite edge to her voice now. "But now he's in mine. Are we clear?"

"Touche," thought Castle to himself.

A pause. "Clear," Beckett agreed. Castle knew she hated to be forced into backing down, ooh she was going to be pissed. Sure enough, when the door opened, she stormed out of the office looking murderous. Members of both teams leaped out of her way as she advanced on Castle.

"I need coffee," she said, through gritted teeth. He pointed towards the breakroom and she immediately stalked off. It wasn't often he saw her so rattled; she had certainly met her match in Teresa Lisbon.

Beside him, Jane let out a long, low whistle.

"Whoa," he said. "Is she always like that?"

Castle nodded. "Pretty much." He hoped it would never change. She was so sexy when she was mad. "And Lisbon, is she always like that?"

Jane chuckled. "Oh yeah. She's a firecracker, has been since the day I met her. But it was kind of refreshing to see her yelling at someone other than me for once."

"Amen to that," said Castle, and then lowered his voice so only Jane could hear him. "Is it wrong that I found that whole fight totally hot?"

All the players are present and round one of the team leader's struggle for dominance goes to Lisbon, but Beckett will get her own back.

I hope you enjoyed this chapter. If I end up continuing this you can expect continued friction between Lisbon & Beckett, some fluff of both the Jisbon and Caskett persuasion, some case-solving, some ass-kicking, and of course, antics from the two coolest consultants ever!

Please drop me a review and let me know if you think this story is worth pursuing, if not, I'd rather know now.

Thanks for reading.