Castle is hired to uncover if a client's husband is having an affair, but the case takes a shocking turn when Castle witnesses the client's murder - by her husband. But with no proof that the killing took place, Castle and Beckett must investigate to expose the truth.

Stakes rise as the 12th Precinct matches wits with serial killers Jerry Tyson and Dr. Kelly Nieman. With the clock ticking, Castle and the team must crack the case before the killers claim their next victim.