Beckett is still plenty peeved at Castle for looking into her mother's murder against her wishes, yet Captain Montgomery forces her to chat with a reporter who is jazzed to write about her crime-solving partnership with Castle. But when a murder victim is found hanging in a tree, Beckett reluctantly teams with Castle once again. Their investigation takes a strange turn when armed men kidnap the body en route to the coroner, leaving Castle and Beckett to delve into the world of drug smuggling, the Russian mob and high stakes poker in Chinatown.
When Castle and Beckett are handed the murder of a couples therapist and Ryan and Esposito are assgined to investigate the death of a dead man found shot in a park, Castle makes a bet on which pair can solve their murder first. But when both investigations hit dead ends and the victims are found to be strangely connected, the two teams work together to solve the peculiar puzzle.
When a young, up-and-coming model is found stabbed with a strange object in a fountain, Castle and Beckett are forced to explore the cut throat world of fashion modeling. Meanwhile, Alexis reconnects with her old baby-sitter, and Ryan and Esposito discover an amusing secret about Beckett.
While dropping playful hints to Beckett about the plot of Castle's first book based on her, the two investigate the murder of a con man posing as a Arctic explorer for school children. The case takes several interesting turns, fascinating Castle and infuriating Beckett, when they suspect that the victim was actually a CIA operative and may have faked his own death. But when an old CIA contact of Castle's disputes that the victim was ever a spy, everyone wonders just who is being fooled.
A woman who is an illegal immigrant is found dead in a manhole, prompting Castle and Beckett to uncover a dark secret about a successful doctor, and a possible connection between his son, and that of the victim. At the same time, Castle's first book based on Beckett, Heat Wave, is released to rave reviews, and he is offered the writing opportunity of a lifetime, causing him to struggle with the fact that accepting the new gig would end his partnership with Beckett.
As Halloween approaches, Castle and Beckett are assigned to investigate the death of a young man found stabbed in the heart with a wooden stake in a graveyard. To Castle's delight, the two must delve into the world of vampires and werewolves to find the killer.
Castle and Beckett investigate the murder of a singer in a popular punk band whose body was arranged to resemble a scene in one the band's music videos. When the case stumps the two, Alexis comes to their aid, using knowledge gained as a fan of the band.
The investigation into the death of a bike messenger takes a personal turn for Beckett and her colleagues when they discover that the victim was carrying evidence that could exonerate a man jailed years before by Captain Montgomery. Things get even more interesting when they discover that a wealthy candidate for the U.S. Senate from a prominent New York family may have been involved as well.
Castle and Beckett are thrust into the world of ex-cons and escorts after a respected Assistant District Attorney is tossed off a multi-story car parking garage. Further, Castle sympathizes with the plight of a call girl connected to the case, resulting in his making promises that he and Beckett may not be able to keep. Meanwhile, Castle is concerned, and mildly hurt, when Alexis confides a secret to Beckett and not to him.
Castle and Beckett have their hands full looking into the death of a man whose body is found stuffed in a garbage chute when two women arrive to identify the body - one claiming to be the victim's wife, the other his fiancée. Meanwhile, Alexis volunteers at the precinct as part of a civics class project, and works to track down the owner of a packet of photos left over from a case gone cold years ago.
A twist arises in the case of an art dealer found shot to death in his art gallery when a man with amnesia arrives at the station carrying a book with a bullet lodged in it that was fired from the same gun as was used in the murder. To solve the crime, Castle and Beckett have to jog his memory and unravel a possible connection to a high-level Arabian diplomat.
Castle reconnects with a former girlfriend on her wedding day when one of her bridesmaids is found murdered, causing sparks to fly between them once again, and Beckett to have to deal with the possibility of being jealous.
The case of the murder of an enforcer for an Irish gang takes a terribly personal turn when Lanie and a forensic expert friend of Castle's uncover an undeniable connection between the case and Beckett's mother's unsolved murder. As the investigation plays out, Beckett comes to terms not only with her grief for her mother, but with feelings that will impact her relationship with Castle.
As Castle and Beckett investigate the death of a possible serial squatter mixed up in a multi-million dollar heist involving exotic animals and diamonds, Castle tries to hide from Beckett the fact that a newspaper that ranked him as one of the city's most eligible bachelors also linked them romantically. While Castle relishes the moment by agreeing to a date with a woman ranked by the newspaper as one of the city's most eligible bachelorettes, Beckett makes romantic plans of her own with Lanie's help.
A beloved Cuban baseball player is found murdered with a baseball bat requiring Castle and Beckett to explore the high-stakes world of professional sports. Meanwhile, Alexis works to complete a family tree for school and questions Castle about his desire to know more about his own father
The body of a woman covered in caramel sauce is found hanging from playground equipment in a park, prompting the team to venture into the world of bondage and fetish clubs, and plenty of playful banter between Castle and Beckett.
Beckett and Castle team up with experienced FBI Special Agent Jordan Shaw in the hunt for a serial killer with an apparent obsession for 'Nikki Heat', the character Castle based on Beckett in his best-seller Heat Wave. It soon becomes apparent that not only is Beckett jealous of Castle's admiration for Shaw and the FBI's hi-tech gadgets, but that Beckett herself could be the killer's ultimate intended victim. Castle also strikes a deal to make Heat Wave into a movie.
After barely surviving an explosion set by the serial killer, now identified as Scott Dunn, that destroys her apartment, Beckett pushes Shaw's imposed boundaries on her involvement in the investigation, and gets herself removed from the case. But when Dunn kidnaps Shaw to get to Beckett, Beckett takes over the investigation and finds herself relying on Castle's instincts as a mystery writer to save Shaw and herself.
When a museum curator is found dead from having a stone gargoyle dropped on him, Beckett, Castle, and the team must answer the question of whether the killer is a jealous colleague, a jilted lover, the publicity-hungry museum director, or the curse of an ancient mummy whose tomb promises death for anyone who looks upon its face, as Castle couldn't resist doing.
When a legendary talk show host is found dead after Castle appears on his show and he reveals to Castle that his life is in danger, Castle and Beckett work to piece together a timeline of his final hours that could crack the case even though it seems the victim died of natural causes. Meanwhile, Castle's fling with an actress who is up for the part of Nikki Heat in the movie version of Heat Wave, has Beckett jealous.
An investigation into the death of a thief uncovers some painful parts of Esposito's past and results in Beckett making a connection with the handsome robbery detective, Tom Demming, assigned to help with the case.
Castle and Beckett's investigation into the murder of a chef who recently won a reality cooking show, results in her reconnecting with an old high school friend, Madison Queller, who owned the restaurant where the victim worked. And while Beckett's relationship with Demming develops, Castle pursues Madison, but whether for romance or information on Beckett's past for one of his books isn't clear.
Beckett asks Demming to help her and Castle investigate a robbery-homicide, prompting the two men to fall into competition with each other to impress Beckett by proving their own theory in the death correct. As the case develops, it becomes obvious to Castle that, despite his efforts at solving the case, Beckett's relationship with Demming has gotten serious.
The death of a man who was involved in a simulated spy game leads Castle and Beckett to wonder where exactly the game ends and reality begins. Against this backdrop, Castle and Beckett struggle inwardly with their feelings for each other, even though Beckett is officially dating Demming and Castle is severely behind in his writing for the second Nikki Heat book. After receiving advice from family and friends, both come to separate decisions that could make or break their relationship in the future.