When the new season begins, Castle is wrestling with how to repair his relationship with Beckett, while struggling to finish his soon-to-be-published bestseller, Heat Wave. But circumstances force the pair back together to investigate the mysterious murder of a man found dead, tangled in the limbs of a tree.

When two separate murders are committed on the same night, Castle wagers Ryan and Esposito that he and Beckett will solve theirs first. The frenzied race to catch their respective killers and win the bet leads each investigative duo to a likely suspect, only to find that they both have airtight alibis. But bizarre twists in both cases force the two teams to work together to unravel the mind-bending mystery behind each murder.

Castle and Beckett get an inside look at the cutthroat world of the New York fashion industry when they investigate the brutal murder of a young model during Fashion Week.

When an Arctic explorer is killed during a polar expedition, the mysterious circumstances of his death take Castle and Beckett from the penthouses of Park Avenue to the halls of the CIA. But Castle's delight at the case's many twists and turns soon turns to frustration as he realizes that their elusive killer might just be fooling them all.

Castle has a new book agent, Paula Haas, but she talks with him when he has gotten an opportunity that may change his career. The downside is that taking the opportunity would mean the end of his relationship with Beckett. Meanwhile, an unidentified woman is found dead in a manhole.

As Halloween approaches, Castle and Beckett are called to a graveyard where the body of a young man sporting vampire fangs has been discovered with a wooden stake driven through his heart. Their investigation takes them deep into the heart of New York's underground vampire fetish community, where they learn that the motive for the victim's murder may be hiding within the pages of the graphic novel he was writing before his death.

An up-and-coming rock star's body is found in a scene that appears staged from her recent music video.

After a bike messenger, carrying a package with content that is able to free a wrongly convicted man from prison, is brutally murdered, Castle and Beckett need to investigate the case. But they are surprised to find out that the tragic death is linked to another murder case previously investigated by Captain Montgomery. Now, in order to solve the new murder, Castle and Beckett must dig deep and discover all the secrets of a prominent New York family.

Castle and Beckett are thrust into the world of ex-cons and escorts. Meanwhile, Alexis has a private chat with Beckett about something that she's keeping from her father.

When a murdered man is found stuffed in the garbage chute of an apartment building and two women arrive to ID his body — one his wife and the other his fiancee — Castle and Beckett have their hands full keeping the women from killing each other as they work to peel back the layers of the victim's double life. Was it a jilted woman, corporate espionage, or a much darker secret that led to his demise?

Castle and Beckett uncover that a bullet is missing from the crime scene while investigating what happened when an art dealer was gunned down in his own gallery. However, when it turns out there was a witness to the murder, they need to refresh the man's memory, only to find out that he suffers from amnesia and can't even remember who he is.

When Castle and Beckett are called to a wedding to investigate the death of a bridesmaid, Castle is stunned to discover that the soon-to-be bride is his old flame, Kyra, "the one that got away". Complications ensue as Kyra, caught off-guard by Castle's reappearance in her life, wonders if she's marrying the right guy. With the nuptials forestalled and the entire wedding party under suspicion, Beckett questions Castle's ability to be objective about the case. But when it looks to Beckett like Castle and Kyra might rekindle their romance, her judgment may be compromised as well.

While investigating the death of an Irish mobster, Beckett and Castle uncover a local turf war that may be connected to both a late-night infomercial guru and an international drug smuggling operation. But the case turns agonizingly personal when evidence emerges that the killer they're chasing may have had a hand in Beckett's mother's murder.

Mysteries abound when a family returns from vacation to discover a dead man in their daughter's bed. Thinking they're investigating the murder of a squatter, Castle and Beckett soon find themselves unraveling a multi-million dollar heist. Meanwhile, Castle tries to keep Beckett from finding out that they've been romantically linked when he's featured as one of New York's 10 most eligible bachelors.

A beloved former major league baseball player is found dead after a goodwill trip to Cuba. The investigation, which includes running into Joe Torre, leads the detectives into New York's Cuban community to solve the murder. Meanwhile, Alexis' genealogy project forces Castle to consider the identity of his unknown father.

Federal Agent Jordan Shaw, an insightful and accomplished Federal investigator teams up with Castle & Beckett to hunt down a cunning and elusive serial killer.

In the second part of the story begun in "Tick, Tick, Tick," the serial killer remains at large and is taunting Castle, Beckett and Jordan Shaw to catch him

The investigation into the shocking death of a museum curator, crushed by a falling gargoyle, takes a bizarre turn when Castle and Beckett learn that he isn’t the first member of a recent archeological expedition to die. It turns out there was a legend inscribed over the burial chamber the team unearthed warning that “all who gaze upon the face of the mummy are doomed.” Was the curator just the latest victim of “The Mummy’s Curse”? And if so, what does that mean for Castle, who snuck a peek at the mummy when no one was looking?

When Castle appears on a late-night talk show to promote his book, Heat Wave, legendary host Bobby Mann leans in during a commercial break and whispers, "They want me dead." When Mann dies later that night, seemingly of natural causes, Castle has to convince Beckett that there was foul play. During the investigation, Castle is seduced by a Hollywood hottie, who was the other guest on Bobby Mann's show the night he died.

Investigating the murder of a thief, Castle and Beckett are surprised to learn that Esposito has a personal connection to the suspected killer. Another surprise is in line for Castle when Beckett hits it off with the handsome Robbery detective assigned to the case, Tom Demming. Now this romantic triangle must work together to solve the case

A high profile chef is found frozen to death in the kitchen of a hot New York restaurant. Complications ensue when Castle and the restaurant owner, an old high school girlfriend of Beckett's, share an attraction. Meanwhile, Beckett's relationship with Det. Demming continues to grow.

A robbery-homicide prompts Beckett to invite Demming to assist on a case, giving Castle a ringside seat to their budding romance. When it becomes clear that the two men have very different ideas about how to proceed, the investigation becomes an unspoken competition for Beckett's attention, one in which each man races to solve the case, as Beckett referees between her two admirers.

Nothing is as it seems when Castle and Beckett investigate what appears to be the assassination of an intelligence operative. Meanwhile, Beckett's romance with Demming threatens her relationship with Castle, forcing them to confront their feelings for each other.