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

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.

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.

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.

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

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.