Slime Monsters from Outer Space. No, Really!
A/N: This was written for Darkest Night 2017, a darkfic themed exchange, for debirlfan to the prompt of "This time, Castle's wild theory is right (unfortunately)."
"Mutant killer snails, Castle? Really, that's what you're going with?" Beckett literally shakes her head at her partner's latest bizarre theory and scrutinizes the two whiteboards set up side by side in front of her. Two victims and nothing much to go on, and Castle's got killer snails. Great.
She used to think he came up with these things just to try and crack her professional facade on her particularly bad days. Sometimes that definitely is his aim, but even now, well after she's admitted she finds his theories funny and she's not truly frustrated yet, they still continue to get weirder.
"I'm telling you, Beckett, I think I'm really on to something this time!" Her disapproval clearly isn't even putting a dent in his enthusiasm. She probably shouldn't even humor him when his theories get this weird, but she responds to him anyway. She's not coming up with any ideas herself, and vic #2's family isn't due to come in and talk to them for another half hour.
"Castle. Lanie ran a tox screen. It came back clean in both cases. The two victims being at the same party at the same French restaurant and having the escargot is just a coincidence. How about trying to help me come up with an actual connection here?"
Castle makes a pouting face at her but finally, thankfully, drops the subject. He sadly doesn't help her come up with a better theory before the family arrives. She has to admit she almost misses the distraction, left staring blankly with no new ideas for the entire time they wait. Maybe Castle does know what he's doing with the crazy theories.
Of course, three days later when the victims have disappeared from the morgue, Castle has to bring it up again. "No, Beckett, you don't get it. They weren't spoiled snails that had gone bad, they were mutant snails. Like out of a sci-fi b movie. Our victims are now sadly shuffling through the New York sewers as snail-controlled zombies."
She doesn't even acknowledge that one by letting him get a rise out of her. Instead she tries to derail him by suggesting other odd reasons the bodies could have disappeared. She even throws out a couple truly nonsense theories of her own, but he's apparently decided to stick to the snails just to annoy her.
When there's another unconnected death two days later and Castle claims to have found, of all things, a snail shell in the woman's apartment? Well, Kate isn't proud of herself for it, but she completely loses it at him. There's being obsessed with the macabre and there's being an absolute jackass. Castle usually knows the difference, but faking evidence over one of his theories that he's dragging out way beyond the point of being funny is over the line.
"Castle! If I hear one more word about snails, I am tossing you off this case! I don't have the patience for your distractions when we have three open, unsolved murders on our plate. Enough, okay?"
"But Beckett-"
"Enough, Castle." She holds up a hand between them to stop him from saying anything further. "I mean it."
She isn't ready to apologize when she herself is the one to find a snail shell at the home of their next murder victim. Who, despite Lanie's vigilance, also manages to disappear from the morgue without a trace a few days after they bring the body in. Not only is this case getting frustrating, the bodies are starting to pile up at an alarming rate (unfortunately, only metaphorically since they also keep mysteriously disappearing), and she still doesn't have a better theory than Castle's weird nonsense about mutant snail zombies.
Beckett actually wonders why the Feds haven't been called in, but aside from something that cannot be real, there isn't any connection between the cases. Just a sudden upswing of murders crossing her team's desk that she feels further and further away from solving with each day that passes.
By the time she finds the fifth snail shell, she starts to wonder if it's just cases coming to her team. Despite the mounting pile of shells in evidence, she still doesn't know how to ask her boss or the other detectives without sounding crazy. They can't really have some kind of serial killer who leaves snail shells behind at the scene, can they?
In the end, solving the case turns out to be the least of her problems. Three days after the last strange, unsolvable murder, all of her victims and an alarmingly large shambling horde of others besides do actually reemerge from the sewers in the middle of downtown New York, stopping traffic and attacking people. Acting like actual zombies, clearly dead, and dripping an appallingly smelly greenish gray slime on top of it. The ones they manage to take down by effectively blowing them apart have some kind of internal large muscular thing controlling them that Lanie identifies as nothing she's ever seen before – but closest to a snail as anything. And they just keep coming. How many of them can there possibly be down there?
When the spaceships show up overhead a week later and she realizes they're alien snail-controlled zombies, she's past even being surprised anymore. She still can't help turning to Castle, who hasn't said a word about snails, not even the 'I told you so' she'd expected, since Lanie's autopsy of the snail-like creature.
"Mutant killer snail zombies, Castle. Seriously, this is the crazy theory you get right. You couldn't have been right about the Men in Black? Or the crazy CIA conspiracies? It couldn't have been, I dunno, when you were going on about evil twins?"
"C'mon, Beckett, this is totally not my fault!" Castle's face is hilariously wounded from the very suggestion. It's almost enough to make her crack a smile, despite the current situation.
As they both get ready to try and take out yet more of the snail zombies from their relatively safe position behind a barricade, she replies, "I don't care. I'm still blaming you for even saying it."