Amy finds herself on the wrong side of the law, and Kirsten puts herself in danger to keep Amy safe and get to the truth - leading to the hardest choice of their careers.
The mysterious disappearance of a Scottish fishing trawler and a passing on-board a Trident nuclear submarine bring the police into conflict with the Navy and British security services. DCI Amy Silva leads an investigation on land and at sea into a conspiracy that threatens the very heart of Britain's nuclear deterrent.