A man in a prisoner’s outfit is shoved into a tree branch and killed. Beckett comes to the scene. Esposito and Ryan tell her that despite the prisoner’s outfit, his finger prints are not in the system.
Castle is depressed over Beckett leaving him. He goes to Lanie to ask if she knows what’s going on but Lanie backs Beckett, saying that Castle disappeared for two months last summer. Castle points out that he was taken, Beckett has walked out on her own. Lanie only tells him to give Beckett the space she’s asked for.
The murder victim is Peter, a college student at Hudson University. Castle noticed his Hudson Red Devil tattoo while he was at the morgue. Despite Beckett telling him to stay out of it, he decides to solve the case in order to win her back. He volunteers to be faculty at the University to get near the prime suspect, Scott Powell who terrorized a pledge group and had altercations with Peter.
Alexis poses as a coed and dresses as a sexy angel to get near Scott. Castle tries to intervene and plays beer pong against Scott, but only manages to get drunk before Beckett shows up and puts a stop to it.
Scott tells Alexis that Peter had a secret apartment. When Castle, Alexis, Ryan and Esposito arrive their they find a BDSM set up and “I say when it’s over Peter.” scrawled in lipstick on the mirror.
Castle and Alexis follow the clue and find an underground prison filled with college students as both inmates and guards. Castle gets them out and then brings in Dr. Lilstrom who was in charge of the experiment, to the 12th in handcuffs. Unfortunately, when the police arrive at the “prison” everyone is gone and it’s been wiped clean.
Castle feels guilty for letting evidence become compromised because he wanted to impress Beckett. She goes to the prison with him and they figure out how he broke out of his own prison cell the night he was murdered but realize he couldn’t do it along.
Turns out his cell mate, Emily, helped him break out but then snapped under the pressure of playing the prison game and killed him.
Beckett is working with Vikram to figure out who is behind the big conspiracy. She has Vikram track down a presumed dead Mr. Smith. Smith tells her the drugs that Bracken was moving are still being trafficked and that they have their own unique signature. He also tells her she’s going to be killed if she continues this investigation.
Beckett tells Vikram she didn’t realize this would be so hard. He give her an out but she doesn’t take it.