(Part 1). Castle and Beckett Investigate the murder of a New York taxi driver who was shot to death, and his taxi stripped of its parts. As they search for his killer, they are led to a storage with trace amounts of radioactive material and evidence that there may be a terrorist group building a dirty bomb. Beckett and the squad must cooperate with a ruthless Homeland Security agent Mark Fallon (Adrian Pasdar), but Castle senses that they have been set up. As Castle and Beckett close in on a suspect, they are trapped in a freezer while a nuclear device is loose in the city.