Season 5 of Castle is set to premiere on September 24, 2012. Originally, it was to include 23 episodes, but late in the season, ABC ordered one additional episode.
As Castle and Beckett adjust to being a couple, obstacles loom in their path, including the conspiracy behind Beckett's mother's murder, attractive female reporters and handsome British cops, disagreeing parents, the hard-nosed Captain Gates, and their own personal doubts and fears. Meanwhile, Alexis begins college at Columbia University, Ryan and his wife work at having a baby, and Esposito and Lanie get back together.
- Castle and Beckett uncover the identity of the man behind the conspiracy that killed her mother: Senator William Bracken. Beckett strikes a deal with him that she won't go public with what she knows about him if he promises to stop trying to kill her.
- Beckett takes back her resignation and serves a suspension.
- Ryan discovers that Castle and Beckett are together while they investigate a murder at his house in the Hamptons.
- Castle is implicated in a murder which proves to be a set up by the serial killer, 3XK, leaving Beckett to fight to save his life and prove his innocence.
- The team is featured in a documentary when the leader of an up-and-coming band is murdered.
- Castle and Beckett introduce their parents to each other and become worried when they don't hit it off.
- Alexis moves into her dorm room at Columbia University.
- Castle and Beckett spend their first Christmas as a couple.
- Castle's ex-wife, Meredith, visits to take care of a sick Alexis and plants doubt in Beckett's mind about the future of her new relationship.
- Esposito takes care of a young boy who is connected to their latest victim and, to set him on the straight-and-narrow, shares a bit about his own past.
- Beckett is forced to protect Senator Bracken when he becomes the target of an assassin.
- Castle and Beckett spend their first Valentine's Day as a couple, with Beckett designating a drawer in her bedroom for him.
- Alexis is kidnapped and Castle meets his father, a CIA spy going by the name Jackson Hunt.
- Ryan must re-assume his identity as a drug runner in an Irish mob family, reuniting him with a former flame, to solve a murder.
- Ryan and Jenny decide to try to have a baby, and eventually are successful.
- Beckett rejects the advances of a wealthy, handsome businessman who is apparently the target of a killer.
- Beckett is approached about a job with the U.S. Attorney General's office and takes the offer seriously, since Castle seems to be taking her for granted.
- Castle agrees to let Alexis go on a trip to Costa Rica, despite misgivings about her safety.
- Castle proposes to Beckett after finding out she kept her interview with the U.S. Attorney General's office from him.
Weird Crime Scenes
- A woman is found tacked to her apartment ceiling.
- The star of a cancelled science-fiction television show is murdered at a Comic Con.
- Castle and Beckett find themselves wandering the streets of New York with a witness to a vicious murder, without a method of contacting anyone.
- The team investigates the apparent death of Santa Clause when he falls from the sky and lands in a park.
- A night club owner is strangled to death with a woman's brassiere.
- The team investigate the death of two people who Castle fears are the victims of a cursed video.
- A baker is found dead in a vat of cake batter.
- Castle and Beckett investigate the death of a woman, who appears to have been killed by Bigfoot.
- While investigating two bomb explosions, Beckett accidentally steps on a motion-sensitive bomb, requiring her to stay still while Castle recalls how their relationship started.
- A man is killed when his car is blown up, apparently by a missile fired by an unmanned drone.
- A woman is found floating in the water tower to a seedy motel.