When an ex-con is mysteriously slain in an alley, Castle and Beckett believe a vigilante is behind the murder. But efforts to identify their suspect are thwarted when they discover that he roams the city in a costume — and may indeed be a Superhero. Can they capture and unmask the killer before he strikes again?
A murder scene complete with lots of blood but no victim and high tech life-extension science are all part of the investigation as Castle and Beckett try to solve a murder without a body. Meanwhile, Alexis waits to hear news about her application to Stanford.
While investigating the mysterious shooting death of a young woman, Castle and Beckett face a cop’s worst nightmare when they discover that the gun used in the killing is Det. Ryan’s old service weapon – the one that was stolen by the serial killer, 3XK. Now with the clock ticking, the team must work to solve the murder, track down 3XK and retrieve Ryan’s gun before the killer strikes again.
When Castle and Martha are taken hostage by a crew of bank robbers dressed in hospital scrubs and using classic television doctor names, Beckett finds herself on the outside, desperately trying to rescue them — which proves to be even more difficult with a by-the-book hostage negotiator watching her every move. Castle becomes the de facto leader of the hostages, and discovers that the robbery is actually a skillfully orchestrated distraction for an even more sinister plan. But with Castle stuck on the inside and Beckett on the outside, will either be able to foil the bank robbers’ plans before it is too late?
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 Capt. 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.
A sniper is on a killing spree in New York City. In a race against time and having absolutely nothing to go on except a killer with no apparent motive but to instill terror, this could be the team's toughest case yet. Things just get even more complicated when Beckett begins to experience increasingly strong moments of PTSD, which she tries hard to hide from Castle and the others.
Castle and Beckett have disappeared, leaving the rest of the detectives searching for them. Elsewhere, Castle and Beckett find themselves handcuffed together in bed in a locked room but have no idea how they got there.
A woman is found dead in a car from City Hall, last used by Mayor Weldon. As Castle remains objective to the Mayor's involvement in the case due to his friendship with him, Captain Gates urges Beckett to take him off of the investigation. Then things take a dark turn when Castle is informed that the case linked to Johanna Beckett's murder.
A famous dog trainer is killed, strangled by a dog leash in his hotel room. But the case gets a lot more peculiar when the avenues of investigation lead to a secret dog breeding program, drugs and strangest of all to TV reality star Kay Cappuccio.
Castle obtains a diary, connected to a double homicide that occurred 65 years ago and the murder of a treasure hunter in the modern day. With the aid of the diary, that tells the story of a hard-boiled detective and his romance with a gangster's lover, he and Beckett search find the identity of the killer and the answer to the mystery of the enigmatic Blue Butterfly.
(Part I) A killer Castle and Beckett are searching for is found out to be a part of an international conspiracy; and Castle reconnects with a gorgeous CIA operative. Meanwhile, Alexis finds an internship which Castle is less than pleased about.
A woman dressed as Little Red Riding Hood is found dead in Central Park, wrapped in a red cloak with beastly claw marks on her body. And as more faiytale murders follow, Castle and Beckett follow a series of lies and dark secrets to find the killer before he takes away another happy ending.
When Odette Morton, a contestant on the hit competition show "A Night of Dance," is found murdered in her dressing room, Castle and Beckett must navigate the case's many twists and twirls without stepping on each other's toes. But as they delve into Odette's past, they discover that this reality starlet was harboring a shocking secret that may have led to her death.
When a bomb explodes at a protest rally killing five people, Castle & Beckett realize the key to solving the case is to reconstruct the 47 seconds prior to the explosion. Using video and witness accounts, they uncover the shocking truth about who planted the bomb.
A devilishly handsome detective from Scotland Yard, Colin Hunt, teams up with Castle & Beckett to find the person responsible for the murder of his friend's daughter. Also, Charles Shaughnessy plays Nigel Wyndham, the Deputy General at the British Consulate who Castle and Beckett suspect is up to more than just ambassadorial balls and diplomatic luncheons. Meanwhile, Lanie convinces Beckett that she needs to make a move on Castle before it is too late.
With tensions rising between Castle and Beckett, Castle finds himself a new partner — Detective Ethan Slaughter (guest star Adam Baldwin, Firefly), a tough and colorful cop from the city's gang unit. But Castle gets more than he bargained for when Slaughter shows him a grittier side of police work that puts Castle in the line of fire like never before.
When Castle and Beckett investigate the murder of a man with human bite marks on his body, Castle's wild theories start flying. But none are as wild as what their only witness insists happened - a Zombie attack. As the evidence pointing towards an undead assailant piles up, the team plunges into New York's Zombie subculture to find the killer and bring him in -- dead, undead or alive.
When the murder of an Army veteran puts Beckett on the trail of the man who shot her, Castle must decide how much he's willing to sacrifice to keep her safe. As secrets are revealed and feelings are put on the table, the lives of the detectives at the 12th Precinct may never be the same.