The sixth season of Castle premiered on September 23, 2013 on the network, ABC in the United States. Castle's mid-season finale was November 25, 2013 before coming back in the new year on January 6, 2014
Last season ended with Beckett wrestling over whether to take a high-profile job in DC -- a dilemma made more complicated when Castle proposed to her. In the Season 6 Premiere of “Castle,” Castle and Beckett will deal with the consequences of their decisions against the backdrop of a high-stakes investigation.
Continuing from the season premiere episode, Beckett races against time to find a stolen toxin capable of killing tens of thousands of people. To make matters worse, Castle was exposed, leaving him with less than 24 hours to live.
With Beckett still in D.C., Castle and the boys investigate the murder of former child star Charlie Reynolds, who rose to fame playing the lovable nerd on an early 90s High School sitcom. But the case takes a turn for the strange when Agents Beckett and McCord arrive from Washington to take over the investigation, creating competitive tension in the precinct. But why would a washed-up actor's death warrant Federal scrutiny? Whatever the reason, Beckett and McCord aren't telling. Meanwhile, Castle has to contend with a new face at the 12th, NYPD Detective Frank "Sully" Sullivan.
A grisly murder investigation leads Castle and Beckett to a suspect (Joshua Gomez, “Chuck,” guest stars as Simon Doyle) who claims he's traveled back in time to stop terrible events from unfolding, events that will change the course of human history. Is he simply a deluded killer, or is it possible that he's telling the truth.
Castle and Beckett investigate the bizarre murder of a young woman who was found ritually posed. When the team finds a collection of strange symbols in her apartment and the prime suspect turns out to be a mysterious monk, Castle becomes convinced they’ve stumbled onto a “Da Vinci Code” style conspiracy. Meanwhile, Castle tries to come to grips with the fact that Alexis and Pi have moved in together.
Alexis enlists Castle’s help on an Innocence Review case to prove that a death row inmate, Frank Henson (guest star James Carpinello, “The Good Wife”), was wrongly convicted. With only 72 hours left before his execution, Castle and Alexis (with help from Beckett) urgently investigate the original murder, only to uncover explosive secrets that may seal Frank’s fate. Joelle Carter (“Justified”) guest stars as Frank’s long-time girlfriend who has always believed in him.
When a famed relationship therapist is murdered, it appears that her access to the secrets of her elite and powerful clients led to her death. But when Castle and Beckett discover the victim was in possession of a rare object worth millions of dollars, they realize that this case may be even more complicated – and dangerous – than they thought.
Castle and Beckett arrive at a crime scene to discover that the female victim bears a shocking resemblance to Medical Examiner Lanie Parish. As the team tries to make sense of this mystery, they realize the killer may be targeting detectives of the 12th Precinct.
A bleeding man stumbles into a church, but just before dying, he hands a mysterious bundle to the priest. When the bundle turns out to be a smiling baby boy, Castle and Beckett find themselves accidental nannies to the newborn. But when they determine the baby and the victim were not related, they discover that finding his rightful parents may be just as hard as solving the murder.
When Castle and Beckett discover that the victim of a building fire was actually shot to death, they get on the trail of a serial arsonist turned murderer. Meanwhile, Ryan’s wife, Jenny, goes into labor, but disaster threatens to ruin the joyous moment.
The murder of a young video store clerk takes a strange turn when the team discovers his past history as a high-level hacker. But it’s nothing compared to the shock Castle receives when their investigation leads them straight to his father, Jackson Hunt.
When a rising young pop star is found dead in an alley, Castle and Beckett delve into the victim’s out-of-control hard-partying lifestyle, but as they dig deeper, a shocking revelation threatens to rock their investigation.
When the assistant to Modern Fashion Magazine’s dictatorial editor Matilda King is found murdered, Castle and Beckett are thrust into the glitzy, ruthless world of high fashion in their search for the killer. Meanwhile Beckett’s history in the modelling world gets her access to a very special wedding surprise.
The murder of a high school mean girl draws Castle back to one of his many alma maters, where he is giddy to learn that the evidence points to a surprising perpetrator – a teenage telekinetic, throwing he and Beckett into a real-life “Carrie”.
When a guilt-ridden young woman confesses to the murder of a struggling actor -- knowing details only the killer could know -- Beckett and Castle seem to have an open-and-shut case, until they uncover proof that their suspect couldn’t possibly be the killer. When a second person confesses, equally convinced they committed the murder, the team realizes the case is far more mysterious than they thought.
Beckett is recruited to assist on a Narcotics investigation, but a routine mission turns deadly when an undercover operation goes array. To survive, Beckett must engage in a battle of wits against some of the most dangerous men in the New York underworld – including a powerful enemy from her past.
When a Japanese ballet dancer is mysteriously murdered, Castle and Beckett’s investigation into her secret life uncovers evidence that she was killed by a ninja, and now that ninja is coming after them.
Castle and Beckett investigate the murder of a Wall Street trader, but the case takes a shocking turn when the they discover the victim was an undercover informant for the U.S. Attorney’s office. To complicate matters further, the U.S. attorney involved is Elizabeth Weston (guest star Salli Richardson-Whitfield, “Eureka”), Captain Gates’ estranged sister.
When construction workers find the body of an infamous New York mobster who disappeared in the seventies, Beckett and Castle try to solve this three decade-old Mafia hit. Unfortunately, their only witness is a man who thinks it’s still the 1970’s, so to uncover what he knows, they must play into his delusion, and party like it’s the last days of disco.
When an up-and-coming professional skateboarder is gunned down by a mysterious motorcyclist, Castle and Beckett are thrust into the world of extreme sports, where the death defying stunts aren’t the only thing that’s dangerous.
A secret investigation conducted by Beckett related to her mother's murder goes awry when a man she has been surveilling is killed. When evidence links Beckett to the victim, she and Castle become the targets of a manhunt that puts both of them in grave danger.
Castle and Beckett's long-awaited wedding is almost here, but when a shocking surprise threatens to ruin the big day, they are sent on a wild mission filled with masked men, angry bikers and a charming ne'er-do-well from Beckett's past (guest star Eddie McClintock, “Warehouse 13”).
For the first time since the show began, Molly who plays Alexis Castle is now being credited as Molly C. Quinn but in previous seasons she was credited as Molly Quinn.