Castle returns to Los Angeles in search of info about the missing time related to his abduction, and while he's there, the Greatest Detective Society reaches out to him and asks for his assistance in solving a murder.
A student learning to speak English is killed and his classmates keep mum when questioned by police, so Castle goes undercover to gain info on the victim by enrolling in the class as a French Canadian immigrant.
A seductive con man is killed, and the investigation has Castle and Beckett questioning New York City's upper crust. Meanwhile, Castle has a surprise in store for his and Beckett's one-year anniversary as husband and wife.
Det. Slaughter returns seeking Castle's help in a robbery case. However, Slaughter becomes a suspect when the heist case turns into a murder case, and Castle's unsure about whether his pal is innocent or guilty.
Castle goes undercover as a professor after a college fraternity student is killed in a grisly manner. During the investigation, he and Beckett unearth some sinister secrets the university wants to keep suppressed.