I thoroughly enjoyed this episode. It was creepy, it was intense, and it made me excited for the extended story line of the season for the first time since Beckett left her DC job. Let's go through the plot.
Our murder victim was found strung up by clear fishing line, her feet just above the ground. That gave the illusion that she was just standing on the ground, rather than actually hanged. That's not the only thing that makes this victim creepy: she looks just like Lanie. As the team starts to look into the case, we learn that the victim has gotten plastic surgery to make her look more like Lanie, and even has a tattoo identical to Lanie's. As the investigation continues, the team is led to a second victim, strung up just like the first, only this one looks just like Esposito.
They find a connection in the form of a plastic surgeon (Kelly Neiman) who knew both of the victims, but she has no clear motive, and anyway she has an alibi. Castle begins to suspect that this has something to do with 3XK. Of course, everyone believes him to be dead, but when Castle and Beckett go to find the 3XK files, they find they've all been checked out... by Esposito. But wait... medical files on 3XK have been checked out by Lanie. Turns out, the lookalike victims are involved. They then track down a man named Carl Matthews, who seems to be tied up in this somehow. Where do they find him? Well, the motel where they had earlier tracked down 3XK. Seems like one hell of a coincidence. Matthews confesses to several murders, but he does not explicitly mention a connection between himself and 3XK.
Then, a shocking new piece of evidence comes to light: Kelly Neiman was the doctor who worked on 3XK when he was in prison! When Beckett and Castle arrive at Neiman's office, they find it totally wiped out. All that's left is a pen and a note that says "better luck next time." But it keeps getting weirder - the pen is actually a thumb drive, and it contains an audio file of Vera Lynn's "We'll Meet Again." Dun, dun, dun...
So, wow. This was seriously an excellent episode. Definitely the best of the season, that's for sure. I was genuinely spooked by a lot of the stuff going on. The creepy setup for the murder victims was something else. And the fact that they looked like Lanie and Espo was really chilling. There was a really strong moment where Ryan saw the second victim and he frantically told Espo not to look, but Esposito comes and sees his own face staring back at him.
Then we've got the 3XK thing. He was always the scariest villain this show had to offer, and I'm unbelievably happy that they've decided to bring him back. The most chilling moment for me was when they realized all of the files were missing, and that the threat of 3XK was very much present. Then the ending, with "We'll Meet Again," was really strong.
Character-wise, this episode actually shone a spotlight on Esposito and Lanie, which I thought was interesting. If I had to point to a flaw, I would say that I wanted a bit more about their relationship, and a bit less that focused on the mystery of it all. Obviously, they've both now seen corpses that look frighteningly similar to themselves, and that's bound to be profoundly upsetting. At the same time, they have seen each other "dead" now as well, which puts a strain on their romantic relationship. Lanie has always been a favorite of mine, so I'm glad they're taking a closer look at her, and Esposito is always fascinating to focus on.
I would say a lot more, but honestly I don't have anything to complain about, and I'd rather just sort of bask in the awesomeness of this episode. It had a great idea, it was creepy, dark, introduced a properly menacing conflict... wow, I just cannot say enough about how much I enjoyed this one.