When a crew member is found dead aboard the Trident nuclear submarine HMS Vigil, the Scottish Police are called on to investigate. However, the UK's nuclear deterrent must remain unbroken, so the submarine stays on patrol.

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.

Kirsten believes the murders were part of a coordinated attack. Her investigation takes her into the heart of Scottish politics and the naval cover up of a near catastrophe in the US.

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.

Vigil is forced into lockdown as disaster strikes. With hostile boats tracking the submarine, and communications down, Vigil's crew have to navigate this crisis alone. Kirsten desperately hunts for clues to reveal the source of the threat to Vigil.

Amy is trapped and running out of time; on land, Kirsten chases down the traitor's accomplice; when the traitor on Vigil puts their plans into action, the crew are forced to fight for their lives.