In a brand-new investigation, DCI Amy Silva and DI Kirsten Longacre enter the world of drone warfare to catch a killer. Following multiple unexplained fatalities at a Scottish military facility, Silva and Longacre are tasked with uncovering the cause. Entering the hostile and closed ranks of the air force, the pair must face the deadly warfare of tomorrow as they fight for their own future.

When a crew member is found dead on board the Trident nuclear submarine HMS Vigil, police in Scotland are called in to investigate. The catch? The UK's nuclear deterrent must remain unbroken, so the submarine stays on patrol and Detective Chief Inspector Amy Silva must go aboard to begin an investigation.

Although the death was written off as an accidental overdose, Amy suspects foul play. But when the crew close ranks in the face of Amy's questioning, a new threat overshadows her inquiry.