Castle and Beckett investigate the murder of a women found frozen and tangled in steel bars at a construction site. The case leads them to look into an old case and also has Beckett thinking about some memories of her past.

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.

When the detective who investigated Beckett's mother's murder is shot before Beckett's eyes before he can give her new evidence in the case, Beckett's old wounds are reopened. The investigation of the detective's death leads the team to an old scheme which may provide the answer to why Beckett's mother was murdered and who ordered it. Beckett and Castle's relationship takes a new turn.

Kate Beckett's investigation into her mother's death is reactivated when Hal Lockwood - a professional hitman with knowledge of Beckett's mother's murder - stages a spectacular escape during a court hearing. As police launch a manhunt for Lockwood, Castle and the team uncover evidence of a deeper conspiracy involving a mysterious figure with law enforcement connections who is linked to both Lockwood's escape and the killing of Johanna Beckett. The search for Lockwood and the quest to identify his co-conspirator leads to a shocking and deadly conclusion.

Detective Beckett fights for her life as Castle, plagued by guilt over his role in the events, struggles to uncover who's behind her brutal shooting. Meanwhile, back at the precinct, Ryan and Esposito have to adjust, as Victoria "Iron" Gates becomes the precinct's new uncompromising Captain.

In the aftermath of a disastrous "Meet the Parents" dinner, a bickering Castle and Beckett are sent to retrieve the only witness to a murder, but they get ambushed and are forced to take the witness on the run. Stranded in the middle of the night in a bad part of town, with no phone, badge or gun, Castle and Beckett must find a way to protect him from a team of mobsters -- that is, if they don't kill each other first.

Beckett's life is on the line when she steps on a pressure sensitive bomb. While the team searches for a way to disarm the explosive, Castle distracts Beckett by arguing with her about who fell for whom first.

In the season finale, an interview with a high-profile federal law enforcement agency forces Beckett to reconsider what she wants out of her job and out of her relationship with Castle. Meanwhile, the body of a young woman is found floating inside the water tank of a skid row flop house, but as the team investigate, they discover that her final resting place is just one of many strange things about her death.

A murder investigation launches Castle into what seems to be an alternate universe where he's never met Kate Beckett. Castle has to convince Beckett, Esposito and Ryan -- now complete strangers -- to let him participate in their investigation so he can find his way back to his own world. This inspires Castle & Beckett to finally take a trip to the altar.