Vigil's reactor shutdown leaves the submarine exposed and the crew racing to repair the fault. Kirsten uncovers a feud among the Vigil crewmates, leading Amy to question whether her investigation has been deliberately obstructed. A hard-won breakthrough leads Amy to believe she finally has the killer in her sights, only for surprising new evidence to cast doubt on her theory. On land, Kirsten's investigation leads to tragedy.

Amy confronts one of the crew with her discoveries about their deception, but with her mental health suffering, questions are asked about her fitness to investigate. On land, Kirsten refuses to bow to pressure from the authorities and has concerns that they are orchestrating a cover up. Amy uncovers terrifying new information about someone aboard Vigil. Kirsten closes in on the killer, but their capture reveals the grave threat that Vigil faces.