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 killer, 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 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, but with no word from the boat, she fears for Amy’s safety. Amy’s quest to uncover the truth places her in grave danger.

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.