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.

When a weapons test turns deadly, with allies from the Middle East among the dead, all eyes are on DCI Amy Silva, who tries to ignore the politics to get to the truth.

After Amy and Kirsten battle for their lives against an armed killer, Amy follows a new lead to the Middle East. She discovers someone on the airbase was complicit in the attack.

When an officer is stabbed on base, Eliza warns Amy to get control of her investigation. Meanwhile, Kirsten uncovers key evidence linking a squadron member to the attack.

Amy and Eliza are held hostage by the dissidents at the heart of the investigation, who demand their help to infiltrate the base. Kirsten will do anything to find Amy.

In the aftermath of a brutal rescue mission, Amy is questioning everything. Kirsten begs her to come home, but Amy can’t let go of her suspicions.

Amy finally discovers the full scale of the conspiracy, and she must decide if she is prepared to risk everything to find those responsible.