Castle and Becket are engaged. Becket starts FBI training in Washington DC and is assigned to a case. Someone broke into a secured building and may have stolen codes. Castle flies to DC and exposed to deadly gas. The code theft was a diversion.
Esposito and Ryan investigate a former TV star's murder, but Becket and her boss take over the case and do not want Esposito and Ryan involved. The guys ask Castle to get information from Becket because they want to solve it before the FBI.
A woman wakes to find her boyfriend murdered in her apartment. She eludes police and takes hostages and waits for Castle to help her. Castle believes she has been framed, and works with the team to prove her innocence.
A grisly murder investigation leads Castle and Beckett to a suspect who claims he's traveled back in time to stop terrible events from unfolding, events that will change the course of human history. Is he simply a deluded killer, or is it possible that he's telling the truth.
The team investigates the ritual murder of a woman. Symbols on the woman's body and the items left in her apartment seem reminiscent of The Da Vinci Code. Alexis and Pi have moved to their own apartment. Castle finds it hard to accept.
Alexis volunteers in a criminal review organization to help prove a man was wrongly accused of murder 15 years ago. With his execution scheduled in three days, Alexis asks her father to bring a new look at the evidence.
A man dies inside a church, protecting a baby. The team has to find the guy's killer and the baby's parents. The man is connected to a crime, but his protecting the baby does not fit a criminal character.