Castle continues to deal with the fallout of Beckett's departure as she and Vikram continue to dig up leads on Loksat. Meanwhile, a PhD student is found murdered in Central Park, and sensing an opportunity to win Beckett back, Castle involves himself in the case.
- Air-Vent Passageway: How Peter escaped from his cell. The fact that a second person was needed for him to get to the vent is the clue that leads Castle and Beckett to the killer.
- Armor-Piercing Question: Castle gets his inspiration on how to win Beckett back when his new AI assistant asks how they got together to begin with.
- The Bus Came Back: Beckett seeks out Mr. Smyth for advice on how to identify a mysterious individual Vikram found during his investigation.
- "Eureka!" Moment: After figuring out the trick behind Peter escaping from his cell, Castle and Beckett realize that he couldn't have pulled it off without help from the girl in the cell next to him. This leads them to the conclusion that she murdered him after they escaped.
- Held Gaze: Castle and Beckett stare into each other's eyes for a little too long before Castle boosts her up to an air vent.
- Inertial Impalement: The Victim of the Week, Peter, is found impaled on a tree branch after the killer shoved him onto it.
- Only in It for the Money: Emily only agreed to partake in Dr. Lillstrom's Stanford Prison Experiment because she needed the money to pay her tuition, but it soon took a toll on her to the point she no longer cares about that.
- Stanford Prison Experiment: Peter's advisor was secretly running one with support from the US Army. He escaped after being demoted from guard to prisoner, after which he was killed.
- Sympathetic Murderer: After they worked together to escape Dr. Lillstrom's makeshift prison, Emily ended up killing Peter, having discovered the real reason he beat her out for a prestigious scholarship, which rendered all her hard work and the trauma she endured pointless. When confessing, Emily breaks down crying about how the unfairness of that revelation, combined with the experiments, caused her to suffer a mental breakdown and commit the murder in a moment of anger, making it clear she regrets it.
- Teacher/Student Romance: Peter was suspected of having an affair with his doctoral advisor. It turns out that he was actually sleeping with the Dean, which helped him win a scholarship. This in turn motivated the killer after they found out about the affair, angered at being cheated out of a scholarship they desperately needed.
- Totally Radical: Ryan and Esposito attempt to disguise themselves and act like college students in order to get a suspect to talk. It goes about as well as you'd expect. Later, Castle challenges a boy flirting with Alexis to beer pong.
