Season synopsis
Contains spoilers for this season and earlier seasons.
After Beckett ends her relationship with Tom Demming, she plans to tell Castle how she feels, but he leaves for the summer with his ex-wife and publisher, Gina. When Castle returns months later, Beckett arrests him at a murder scene; he helps solve the case and earns his way back into her investigations. Beckett begins dating heart surgeon Josh Davidson, while the tension between her and Castle remains unspoken. In “3XK,” a new murder bears the signature of a serial killer believed to have vanished, bringing that threat back into their work.
Midseason, the murder of a taxi driver leads Castle and Beckett to evidence of a terrorist group preparing a radioactive dirty bomb. Working with Homeland Security, they pursue the threat until the suspects trap them in a freezer while the device is still loose in New York. The NYPD continues searching as Beckett’s boyfriend Josh joins the rescue effort; once they are freed, Castle and Beckett return to the task force and help track the terrorists. This crisis leaves the pair confronting how much they rely on one another, without either one resolving their feelings.
The mother-murder case surges back when Hal Lockwood escapes custody. Montgomery is exposed as one of the three officers involved in the old conspiracy. He tells Beckett he was complicit but withholds the powerful figure’s identity, warning that the knowledge would endanger her. Castle takes her from the scene while Montgomery confronts Lockwood’s men, kills Lockwood and his associates, but is himself mortally wounded in the gunfight. At Montgomery’s funeral, a sniper shoots Beckett in the chest; Castle reaches her and tells her he loves her as she loses consciousness, while the conspiracy’s sponsor remains unidentified.