When a crew member is found dead on Trident nuclear submarine HMS Vigil, Detective Amy Silva is flown onboard 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.

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 communication lost, Kirsten desperately hunts for clues to the source of the threat to Vigil.

Amy is trapped and running out of time. Vigil's crew must identify the traitor or risk tragedy.