Castle and Beckett re-evaluate their relationship after spending the night together.

Beckett returns from her suspension while she and Castle keep their relationship hidden.

Wendy Dupree, murdered in a cheap motel, wrote a clue in her blood. When Castle realizes it's no word but a number, it leads, in combination with her phone records, to a storage compartment, about to be auctioned, rented by her brother Wendell, an ex-con, who died in a suspicious accident a few months earlier. He was employed as driver by socialite Anjelica Henley, who recently reported a super-valuable diamond, stolen in a robbery, focus of further criminal intrigues. The earlier past provides the crucial clues, while Castle and his mother worry about Alexis's impending moving out to a dorm, albeit in New York.

Castle and Beckett's romantic weekend in the Hamptons turns sour when a dying man stumbles into Castle's pool.

After Tessa Horton is murdered in a bizarre fashion at her apartment, evidence piles up, all pointing at Castle. He denies knowing her, let alone having had an affair, but there are fingerprints, security footage and recovered hard-drive evidence all showing he did. Although no one in the precinct can believe him guilty, they must arrest him based on the evidence. Only then does vindictive serial killer Jerry Tyson manifest himself to Castle while he is in Holding at the precinct, promising that he won't survive jail, which calls for a radical solution.

During the annual major science fiction convention, Nebula 9 - super-fan Audrey O'Neill, who finally organized a life performance starring the short-lived series' original cast as well after decade of 'webipsodes' with a nerd couple, is found dead on set, killed with a real 'space laser' (of which only two working ones were custom-made). Castle, who thinks he has better taste, is amused to find Becket was part of a college fan group. The team works out various personal, career and business motives as well as opportunities for cast and crew.

A documentary crew follows Castle, Beckett and the team as they investigate the murder of a rock band's lead guitarist.

When Beckett and Castle pick up a witness to a murder, hired killers attack, and they lose their cell phones and Beckett's gun. They run for protection while Esposito and Ryan try to find them.

A man dressed as Santa falls from the sky while Castle and Beckett prepare for their first Christmas as a couple.

A divorce lawyer is found dead. Meredith visits to take care of a sick Alexis while Beckett is staying there due to her apartment being fumigated.

A promising DJ is murdered at an album-release party for a pop star which takes Castle and Beckett inside the drama of NYC's music industry

Soft porn tycoon Beau Randolph was strangled with a bra during a grand party to launch another College Girls Gone Crazy franchise production. He received many death threats, such as from arch-conservative Ronald Armstrong's grim prudish movement. Beau's own bodyguard Scarlet Jones, whom Javier finds irresistible, stole a memory card from his safe and was hired by children's TV production firm. Actress Tiffany Shaw also lies a lot about her disguised party presence, which is only the top of an iceberg of family and business secrets.

The victim's corpse is found burning in a barrel by a homeless man. The team finds out she worked for US Senator William H. Bracken, whom Beckett holds responsible for her mother's murder. But he seems to be the target, this time. Only Castle learns that Beckett considered holding back evidence before another clue leads the team to prime suspect Robert McManus, a mechanic who blames the senator for his son's suicide. The senator is hell-bent on holding a high-profile speech in a hotel to launch a career-crucial environmental bill.

Hannah Green, star of a reality show, is found dead at a bus stop with a knife in her back. She has bruises from being held captive for the day she went missing from the studio. The team unravels complex rivalries for parts, infidelity and spin-offs that affect both a clothing line and Margo's son Stone Gower's series project with co-star Collette's daughter, Ashley. Castle and Becket fear the captain will discover their affair after he accidentally drops Beckett's Valentine present in the wrong coat.

Hasim Farouk, a student at Columbia like Alexis, is killed by a van crashing into him while he emptied his gun into it. The team soon works out the van was used to kidnap not only Egyptian tycoon Anwar El-Masri's daughter Sara, but also Alexis, whom she attended a lecture with. FBI agent Harris' sole lead, obtained from Castle torturing the wounded van driver, runs into a dead-end. Meanwhile, the girls attempt an escape, but discover they are not where they thought they were.

After the FBI fails to get Alexis back, Castle takes matters into his own hands.

Castle is shaky after watching a DVD which predicts, in flashy amateur horror style, the exact time of death, for no apparent reason and probably by panic-caused cardiac arrest, of the murder victim, screaming in the phone, who received it. Her ex has a solid alibi and couldn't make sense of her panic about 'it'. Tracing the DVD's shipping leads to another copy recipient, killed at home similarly. Their only link is having been jurors on executed country serial killer Nigel Malloy's trial. The team heads for his home town, loved-ones and surviving jurors.

The corpse of Jimmy, a baker popular for cup cakes, is found in his industrial dough mixer. His apprentice, Todd, noticed someone in a vintage muscle car, before being sent away. The car was identified as belonging to Irish mob boss Robert 'Bobby S.' Shannon. After FBI agent Sam Walker tells the team that Jimmy was his informant, Ryan astounds everyone by revealing he had been undercover in that gang years ago. And, to save a friend, chooses to revive his alias, Fenton, and go back undercover.

Castle is home, mostly alone, in a wheelchair after a skiing accident, bored out of his wits just before his birthday. After a while, he actually picks up a pair of binoculars Alexis gave him to play Rear Window. Indeed, right across the street a foxy resident betrays her studly husband with a younger hottie. Shortly after, the cheated husband finds his rival's hat, wields a knife and seems to dispose of the corpse while she isn't seen again. Yet his NYPD team mates only reluctantly investigate and can't find any proof. Castle becomes obsessed with the possibility that he witnessed a murder, while Beckett grows increasingly upset with his mania and sets out to find out the truth one way or the other. That's when things take a surprising and shocking turn...

A woman's body is dumped outside a hospital, with her face torn open as if by an animal or wild beast. Soon, Beckett and Castle become prey for the killer.

Castle is jealous when Beckett has to protect a billionaire from an unknown killer. Castle wants to solve the case fast because it means Beckett will no longer have to be with the charming entrepreneur.

After tracking a killer to his apartment, Beckett steps on a bomb trigger and must stand in place while the bomb squad finds a way to defuse it. Castle stays with her, and they argue over who liked who first, via flashbacks.

When Homeland Security inexplicably seals the crime scene of a car bombing, Castle and Beckett find themselves with two mysteries on their hands: who murdered the victim, a government whistleblower, and why are federal agents trying to take over the case? The plot thickens when they discover that the victim wasn't killed by a car bomb but by a missile from a military drone.

The team investigates the death of a woman who hacked a law office for a specific file. It seems she thought her best friend was killed, and she was on a lead to that killer. Beckett interviews with the FBI.