While taking on a high-stakes investigation, Beckett decides what to do about whether to take a job in DC, and if she should accept Castle's proposal.
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 (Joshua Bitton). Antonio Sabato, Jr. guest stars as Ramon Russo, the victim's former sitcom costar.
A desperate young murder suspect, Emma Briggs, takes hostages at gunpoint then mysteriously insists that she will only negotiate with one person: Richard Castle.
A grisly murder investigation leads Castle and Beckett to a suspect 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, 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.
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.