The discovery of a body aboard the nuclear submarine HMS Vigil prompts the dispatch of Detective Amy Silva to the scene.

As Vigil faces imminent peril, Amy must press onward with her inquiry. Simultaneously, Kirsten's pursuit of answers takes a devastating turn.

Kirsten's inquiry plunges her into the depths of Scottish politics. However, the discovery of a naval cover-up casts a shadow over Vigil, leaving Amy to question the very foundations of trust.

As Kirsten draws closer to the killer, the implications of their capture reveal a profound danger facing Vigil. Meanwhile, aboard the submarine, Amy uncovers a discovery of truly terrifying proportions.

When disaster descends, Vigil is abruptly placed under lockdown. Communication severed, Kirsten frantically searches for answers—a trace of the origin of the danger that now threatens their sanctuary.

Time is dwindling for Amy, and she is caught in a desperate situation. Will help arrive before it's too late? On the ground, Kirsten pursues the traitor's associate. Meanwhile, when the traitor on Vigil initiates their scheme, the crew must…