The mysterious disappearance of a Scottish fishing trawler and a death on-board the fictional Vanguard-class nuclear-powered ballistic missile submarine HMS Vigil bring the police into conflict with the Royal Navy and British security services.
The Scottish police begin an inquiry and Detective Amy Silva (Suranne Jones) is forced to join the crew onboard to conduct her investigation. The Vigil must remain on patrol during the investigation in order to maintain the UK’s nuclear deterrent. Meanwhile, Amy’s ex-girlfriend Detective Kirsten Longacre (Rose Leslie) follows leads back on land. As the two pursue parallel trails, the case grows more sinister and broadens to include the highest levels of government and the British Navy.