When a crew member is found dead onboard the Trident nuclear submarine HMS Vigil, the Scottish Police are called on to investigate. The catch? The UK’s nuclear deterrent must remain effective, so the submarine stays on patrol and Detective Chief Inspector Amy Silva must go aboard to investigate. Although the death has been written off as an accidental overdose, Amy suspects foul play. But the crew close ranks in the face of Amy’s questioning, and a new threat overshadows her inquiry.

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.

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 starts to question who she can trust on Vigil, when a message from Kirsten makes her realise she is being deliberately undermined.