It had begun, like so many other cases, with a dead junkie in an alley and way too many potential causes of death. Things got a bit more interesting when Lanie Parish informed them that their vic had OD'd on some unknown and extremely potent drug. Add to that the fact that their vic wasn't your typical dead-in-an-ally type (in fact, he was a well liked and respected surgeon). An affluent person like that probably wasn't scoring a hit on a street corner, so the pool of dealers got a lot smaller, which was always nice.

Some stellar investigative work and judicious questioning led them to one Elmer (Yeah, Castle had a field day when he found out their skel was actually named Elmer) Green, a chemist whom had decided the designer drug business was far more lucrative (for him personally) than developing a cure for cancer.

Elmer's lab and apartment were in two separate buildings, which was nice because a take down surrounded by crazy ass mad scientist crap was always sketchy. Letting the suited up hazmat guys take the lab, Detective Javier Esposito and his partner Kevin Ryan accompanied Detective Kate Beckett and her constant shadow Rick Castle to Elmer's home.

Ryan had been first into the room and thus was the recipient of a face full of some weird, strangely neon yellow liquid. Blinded, he stumbled off to the side, allowing Beckett and Esposito to pursue Elmer. Esposito gave his partner a worried glance but knew Ryan would want them to take Elmer down, maybe give him an extra hard bounce off the floor for good measure.

As they ducked out of the room after their rabbitting dealer, Esposito caught a glimpse of Castle moving towards Ryan. Though not a cop, the writer was a good guy to have around and Javier knew his partner would be in good hands.

Elmer, as it turned out, was an annoyingly fast little bastard that led them on a chase through the seemingly endless warren of halls and rooms in his obscenely large building (an old warehouse, still undergoing renovations into more apartments). The various pieces of scaffolding, plastic sheeting and random boxes did not help.

Finally, Elmer was on the ground, wheezing and trying to complain as Beckett pressed a knee into his back and Esposito himself was perhaps a touch rough with the cuffs.

They hauled the cursing dirt bag to his feet just as Castle's voice (panicky and disquietingly hysterical) echoed through the building.

"BECKETT! ESPOSITO! NOW!"

It wasn't the clearest message, but the tone and the shriek that followed it, a shriek that would have put the whore hounds of hell to shame, certainly was.

Something was very, very wrong.

When Ryan got a face full of chemicals, courtesy of Elmer (and wasn't that just a foolish name for a drug dealer? Any self respecting dealer would have a much cooler, scarier name, like Gat or Ice Pick ), Beckett and Esposito had done their duty and taken off after the man, leaving Castle to check on Ryan. Honestly, he thought he got the better end of that deal because the oozing gunk dripping down the younger man's face was nasty and Rick didn't want to be on the receiving end of a neon slime facial.

"This is nasty," Ryan sputtered, trying to wipe his eyes clean and spit the sludge from his mouth.

"Did you swallow that?" Rick asked, both alarmed and disgusted. "Spit it out! Spit it out!"

"I'm trying," Ryan began then suddenly went very still.

That was not good, but Castle preferred it to what happened next. Snapping back into motion, Ryan doubled over, dropping to the floor and convulsing.

"Damn it!" Rick exclaimed, dropping to his knees beside Ryan and trying to keep the flailing detective from hurting himself. When the seizure subsided, he pulled out his cell phone but didn't get beyond the 9 in 911 before he froze, staring in horrified, morbid fascination that left him wondering if he himself was having some sort of stroke.

Rick blinked and shook his head, but no, Ryan still appeared to be…he supposed morphing was the word. Skin shifting, features blurring…and oh god, he could feel Ryan's ribcage shifting against his knee! No way could this be happening. Nope. Not possible…but happening.

Only a few seconds after this new, mind and body warping insanity entered the world, it exited but left in its wake some truly terrifying evidence.

Where Kevin Ryan had fallen to the floor, a slight brunette with startling blue eyes lay gasping up at him. Her…his…her… Long hair lay in a tangled, goopy pool beneath her (Rick was going with her for the moment because that was what his eyes saw and it was all very confusing, so the simple answer was grabbed) head and some weird part of his brain wondered where all that hair had come from.

How many laws of nature had just been violated and tossed out the window, right before his eyes?

Then he silently laughed to himself, knowing he was focusing on some small, irrelevant oddity in order to keep his brain intact, cause this was the sort of thing that broke minds.

"Why're you looking at me like that?" Ryan asked, then delicate brows knit. "What's wrong with my voice?"

Waggling a finger to indicate Ryan should be more concerned about what was going on below the neck, Rick found his voice and bellowed, "BECKETT! ESPOSITO! NOW!"

For once, words were failing him but Rick knew the other two detectives would respond to his call.

Failing that, they'd definitly come running when they heard the God awful shriek coming from Ryan's lungs after discovering certain physical differences. Rick was surprised such a small person could make such a big noise. Then again, babies could be exceptionally loud.

Rick hoped they hurried. It really wasn't fair to make him deal with what was essentially a crack! gender swap fanfic made real without any back up. That way could only lead to madness…or a bad foray into the fantasy genre.

Dragging a pissed off Elmer between them, Beckett and Esposito ran back to the apartment, following the echoing screams and Castle's loud reassurances. By the time they staggered back into the room, the ear splitting shrieks had calmed to an occasional shout and Castle was making inarticulate soothing noises while patting the air.

That would have been funny, except for the fact that Ryan seemed to be missing. Instead of the detective, Castle was hovering over a slime covered, delicate, little brunette…who seemed to be wearing Ryan's jacket and ugly green shirt. Weird, Javier thought, then heard Beckett suck in a breath beside him.

The girl on the floor had turned her gaze to them. Startlingly pretty blue eyes peered up at them from a face covered by drying yellow goop, brown tresses sticking to the substance. She opened her mouth and pleaded, "Tell me this isn't happening."

But Javier could only gape. He'd never seen eyes like that on anyone but his partner. His very male partner….

Castle finally stopped patting the air and jabbed a finger in Elmer's direction. "You! What the hell did you throw on him…her…him?"

"Holy shit!" Elmer yelped in reply, staring at the young woman who was struggling into a sitting position. "You're a guy? It worked!"

"Yes," she - were they actually thinking this girl was Ryan? When did his life become a bad sci-fi movie?- grumbled, then demanded, "Fix this!"

To shocked to even obfuscate, Elmer babbled, "I was trying for that result, but when you all showed up I just tossed some random chemicals at you." He indicated the smashed vials and beakers. "I don't know what hit you or in what combination…I couldn't even begin to try to reverse this without careful analysis."

Ryan (never the biggest guy, but now a tiny little thing) glared at Elmer. "So there's really no reason I shouldn't kill you?"

Elmer paled and shrunk back between Esposito and Beckett, hoping they'd protect him from Ryan, whose face was visibly flushing under the yellow slime and her itty bitty fists were clenched in rage.

Javier snorted. He couldn't help it. The situation was too fucking surreal. And, while Ryan was always hot, this version was like some tiny, messy doll.

"I know, right?" Castle said a bit desperately, obviously glad someone else appreciated the sheer lunacy of the situation.

Ryan however did not see the humor. Those eyes snapped over to Esposito and the expression on her face was bone chilling as she stabbed a finger at him. "Not funny, man. Not funny."

Beckett, who had been oddly quiet, finally spoke, "This is not possible."

"Tell that to my new friends," Ryan grumbled, turning her finger around and prodding her chest. "This is so wrong."

"I got it!" Castle exclaimed, tearing his eyes(and everyone else's away from what Ryan was doing) and pointing at Elmer. "The chemicals you tossed around are causing a mass hallucination!"

Javier exchanged a doubtful glance with Beckett, knowing Castle was grasping at straws, but they were willing to go along with it. That idea seemed far saner and Ryan clearly found it the preferable story.

"YES! Yes, let's go with that!" Ryan said, "Call the Paramedics and I'll wake up in the hospital and everything will be back to normal."

That sounded good, but Javier seriously doubted that was going to happen. Then again, if you'd asked him a half hour ago if his partner would suddenly become a woman, he'd have said that couldn't happen too.

TBC….