When a crew member is found dead onboard the Trident nuclear submarine HMS Vigil, the Scottish Police are called on 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 investigate.Although the death has been written off as an accidental overdose, Amy suspects foul play. But the crew close ranks in the face of Amy's questioning, and a new threat overshadows her inquiry.