Castle season 4

After undergoing extensive surgery, Beckett takes three months to recuperate before returning to the NYPD. With Castle in tow she begins searching for her would-be assassin. Captain Montgomery is replaced by Captain Victoria "Iron" Gates, who does not approve of Castle's presence. Although Beckett at first claims that she does not remember the events of the gunshot, she confesses to a psychologist that she does. As Castle tries to dissuade Beckett from digging deeper into the case, he himself is immersed in the process of trying to identify Beckett's shooter.

Castle and Beckett investigate the murder of an ex-con found sliced in half in an alley, and believe a superhero-themed vigilante is behind the murder but the case gets complicated as they find out that there is more than one person dressed up as the same comic book hero.

Castle and Beckett investigate a crime scene that involves a large pool of blood, but no victim, where their desperate search for the body ends at a cryonics company. The victim, a biology professor at Hudson and a friend of the company's CEO, had a cryogenics contract, triggered by an alarm that goes off when the victim's heart stops beating. Taking the bullet out would 'kill' the corpse, so the severed head remains frozen at the cryogenics laboratory. When Ryan and Esposito find needles with the victim's brain matter on it, the case takes a turn when Lanie discovers that someone was doing procedures to the victim's head, which is subsequently stolen during a break-in at the lab. Alexis receives news about Stanford.

Castle and Beckett investigate the murder of a young woman which was committed using Det. Ryan's old service weapon – the one that was stolen by the serial killer, 3XK. The victim's phone records reveal she had been in contact with Seth Carver, an undercover narcotics detective. When looking through old police records, Castle manages to find a link in the case which leads them to Philip Lee, the son of Clifford Lee, a notorious Chinese drug mobster.

Castle and Beckett investigate a murder committed during the theft of a valuable sculpture. Serena Kaye, a smart, sexy insurance investigator, joins them in the investigation, all the while hiding a very important secret from the team. She also attracts a lot of attention from Castle, with mixed feelings from Beckett.

Castle and Beckett investigate the murder of a ghost hunter named Jack Sinclair who was looking into a haunting at a New York mansion. As they delve deeper into the case, Castle supports the idea that supernatural powers could be at work, while Beckett looks for a non-supernatural reason for the murder. While looking at the haunted mansion, Castle and Beckett discover a secret alcove where the killer killed Jack Sinclair, putting Castle's "demon" theory to rest.

Martha and Castle are held hostage by bank robbers dressed in hospital scrubs. Beckett's rescue efforts are hindered by a strict hostage negotiator. Meanwhile, Alexis faces problems in her relationship with Ashley.

During the investigation of the murder of an Atlantic City casino owner at an abandoned warehouse, strong leads emerge for the team in both New York and Atlantic City. As Beckett and Captain Gates work the New York leads together, Castle naturally chooses to join the detectives for the road trip to "America's Playground", hoping to help solve the case while throwing an impromptu bachelor party for Ryan.

The team searches for a sniper who is terrorizing New York; Beckett tries to hide her worsening PTSD from Castle and the detectives.

Castle and Beckett wake up handcuffed together in a sealed room, with no memory of how they got there. While Esposito and Ryan search for them, they must reconstruct their case from what little they remember.

When Castle and Beckett investigate the death of a ladies' man, a secret is revealed about Ryan's bride.

A woman is found dead in the passenger seat of a car on loan from city hall by the Mayor (Derek Webster). Captain Gates urges Beckett to take Castle off the case due to his close friendship with the Mayor.

The investigation into the death of a famous dog trainer leads to reality television star Kay Cappuccio (Hilarie Burton). Alexis goes on a trip with her friend Buttons to look at colleges. Castle and Beckett debate where the dog, Royal, of the deceased (who had no family) should stay. Esposito has a crush.

Castle attempts to solve a crime using a diary written in the 1940s. Every time he reads it he flashes back to the 1940s, with the principal characters being imagined as people described in the diary.

Castle and Beckett find out that the killer they are pursuing is part of an international conspiracy; Alexis starts a new internship assisting Lanie in the Medical Examiner's Office.

Castle and Beckett realize what the linchpin is. While trying to prevent a war from breaking out they are seemingly betrayed by a person they thought they can trust. Throughout the investigation, Castle begins to learn more about his father.

When the bodies of two murdered women are found dressed as fairy tale characters, it falls to Castle and Beckett to catch a killer before he can strike again.

Castle and Beckett take the case of a troubled celebrity who was murdered moments before she was due to appear as a contestant on a popular dancing competition.

Castle, Beckett and the NYPD scramble to find the perpetrator of a bombing at the "Takeover Wall Street" protest. While Beckett is interrogating a suspect, she reveals she has always remembered everything about her shooting, causing Castle (who inadvertently overhears) to think Beckett has rejected his feelings for her.

Castle and Beckett are approached by a detective inspector named Colin Hunt (Brett Tucker) from Scotland Yard looking for help in solving the murder of a family friend. Beckett finally admits her feelings about Castle to Lanie, who convinces Beckett that she must make a move before it is too late.

Feeling that he is no longer able to work with Beckett, Castle finds a new partner in Detective Ethan Slaughter (Adam Baldwin). As the two attempt to prevent a gang war from taking over New York, Slaughter's questionable and often violent methods place Castle's life in danger.

A witness to a murder claims that he and the victim were attacked by a zombie. Castle and Beckett take a step forward in their relationship, with Beckett stating that "Her wall" is about to come down.

When Castle and Beckett investigate the murder of a reformed gang member, they find evidence that links him to Beckett's shooting and the theft of documents from Roy Montgomery's home. The 12th Precinct is divided as Castle tries to stop Beckett from investigating, causing a rift in their partnership. Her efforts are unsuccessful, and Cole Maddox (Tahmoh Penikett)—her would-be assassin—escapes, vowing to kill her once he recovers Montgomery's files on his employer. Esposito and Beckett are suspended for withholding information on the case, however Beckett decides to resign. Beckett then goes to Castle's home where she finally gives in to her feelings for him, and they share a passionate kiss. The episode concludes with the mystery man protecting Kate, identified as Mr. Smith, being threatened by Maddox to give up the information he is using to blackmail them.