When a crew member is found dead on board the Trident nuclear submarine HMS Vigil, the Scottish Police are called on to investigate. However, the UK's nuclear deterrent must remain unbroken, so the submarine stays on patrol.

A small community in Dorset becomes the focus of a murder investigation led by out-of-towner DI Alec Hardy. DS Ellie Miller, who feels the job should have been hers, must now form a professional relationship with the difficult and unyielding Hardy.