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.
A female victim bares a shocking resemblance to examiner Lanie Parish when Castle and Beckett arrive at a crime scene. They soon realize that the killer is targeting 12th precinct detectives.
A priest receives a mysterious bundle when a bleeding man collapses and dies in his church. The bundle turns out to be a baby boy, leading Castle and Beckett to nanny the newborn. Their investigation soon uncovers that the baby and victim aren't related at all and that finding the parents of the now orphaned baby could solve to be as hard as solving the murder case.
A murder victim at a fire scene leads the team onto the trail of a serial arsonist. Ryan and Esposito find more trouble than they bargained for as Ryan's wife Jenny goes into labor.
A seemingly innocuous murder of a simple store clerk leads the team into the world of high stakes espionage, and ultimately, Castle's father.
An out of control pop star is murdered, and the investigation leads Castle and Beckett into a world of paparazzi and debauchery, with Alexis thrown in to boot.
A fashion magazine assistant is found murdered, and Castle and Beckett must wade through the halls of the glitzy fashion world to find the killer.
Castle and Beckett investigate the murder of a high school girl, and find themselves bewildered when the evidence indicates the perpetrator has telekinetic powers.
The murder of a man in a hotel room seems like an easy case when Beckett gets a confession, but due diligence by the NYPD reveals the woman has an alibi, and things get more bizarre when a second and then a third person confesses.
Beckett is requested to join a narcotics sting that is supposed to be straight-forward, but the meet goes bad and she ends up in a fight for her life.
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.