When a crew member is found dead aboard the Trident nuclear submarine HMS Vigil, Detective Amy Silva is flown out to investigate.

With Vigil in danger, Amy must fight to continue her investigation. Meanwhile, Kirsten's enquiries lead to tragedy.

Kirsten's investigation takes her into the heart of Scottish politics, but when she unearths a navy cover-up, it leaves Amy questioning who she can trust on Vigil.

Kirsten closes in on the hugger, but their capture reveals the grave threat that Vigil faces. On the submarine, Amy makes a terrifying discovery.

Vigil is forced into lockdown as disaster strikes. With communication lost, Kirsten desperately hunts for clues to the source of the threat to Vigil.

Amy is trapped and running out of time, but will assistance arrive too late? 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.