The first series stars Suranne Jones, Rose Leslie, Shaun Evans, Paterson Joseph, Gary Lewis and Martin Compston. The second series replaces the nautical setting of the first series with a land-based drama that focuses on prototype drone technology. Detective Chief Inspector Amy Silva of Police Scotland is sent to HMS Vigil, a nuclear-powered Vanguard-class ballistic missile submarine, to investigate a death on board. Her investigations bring the police into conflict with the Royal Navy and MI5, the British Security Service.