When an astronaut training for a trip to Mars is mysteriously killed inside a Mars simulation, Castle and Beckett don spacesuits to investigate. But when they discover no one inside the sealed simulation could have committed the murder and no one from the outside could get in, the case takes a shocking turn.

Detective Kevin Ryan is on the security detail of a charismatic congressman attending a charity event when shots suddenly ring out. Driven to apprehend the killer, Ryan must revisit the events that led up to the shooting, with the help of Castle and Beckett.