Beckett works on her hardest case yet as she waits at the scene of Castle's accident and finds out details about the car crash.
The team take a look at the murder of a toy company CEO and Castle uncovers a lead to his mysterious disappearance which sees him becoming involved in a dangerous investigation of his own.
After a pool shark is killed by an unseen force, Castle and Becket discover evidence that the killer could have paranormal powers. Castle and Beckett try to get their lives back to normal following his disappearance.
An ice-cream vendor is shot and an unidentified schoolchild could be able to help with the case. Castle attempts to find the child by working undercover at an elementary school.
After Castle and Beckett investigate an Internet star's murder, they discover some photos of the crime posted online by the killer.
Castle finds himself in an alternate universe where he's never met Kate Beckett as he investigates a murder. Castle has to convince Kate, Esposito and Ryan to allow him to help in their investigation so he can find his way back home.
When Castle and Beckett learn that a murder victim may have been poisoned at an Old West-style resort, they visit the resort posing as newlyweds to uncover the truth.
When subway passengers, including Detective Esposito, are taken hostage by a mysterious gunman, Castle, Beckett and the team must uncover the gunman's agenda before it’s too late.
When the star of Castle’s favorite '80s action movie is mysteriously murdered, Castle and Beckett delve into the victim’s secret past and pursue his killer with help from a team of aging action heroes.
When an ER physician is gunned down in the street, Castle and Beckett soon discover that the victim was a mob doctor to a Mafia family run by an old friend of Castle's. Also, against the backdrop of Christmas, Esposito and Lanie reach a defining moment in their relationship.
After being forbidden to work with Kate Beckett or the 12th Precinct, Castle obtains a P.I. license and shows up unannounced at Beckett’s crime scene, hoping to investigate alongside her as a private eye. But things don’t go as Castle had planned.
When a young Latin telenovela actress is murdered, Beckett and the team delve into a case that is as flamboyant and full of intrigue as any telenovela. Meanwhile, Castle is hired as private investigator by the show’s sultry star to pursue the case from another angle.
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.