Disappearance of Scottish fishing trawler & death on-board Vanguard class nuclear-powered ballistic missile submarine HMS Vigil causes conflict with police, Royal Navy & British security services. Chief Petty Officer Craig Burke, a member of the crew of nuclear submarine HMS Vigil, reports his concerns over unusual sonar readings while on patrol. Confined to quarters following insubordination, he is later found dead in an apparent suicide. The police are asked to investigate and DCI Amy Silva is airlifted to Vigil's classified location. Unable to contact her colleagues by radio due to the sensitive nature of the patrol mission, she suspects the crewman was murdered, but the captain and other officers disagree. On land, Silva's DS, Kirsten Longacre, has begun a parallel investigation into Burke's movements and his acquaintances. She finds a clue during a search of Burke's room which she keeps hidden from the Royal Navy. It later transpires that Chief Petty Officer Gary Walsh, an engineer on HMS Vigil was in & out of care 'See the scars on his arm his father used to put out cigarettes on his arms.' though what real bearing this has on his personality is unclear - he is shown in a later series as drinking and eventually threatening to kill himself possibly mentioned as a way to justify his flawed character. Whereas Lieutenant Commander Mark Prentice, the Executive Officer and second in command of HMS Vigil who lied about his involvement in the murder just gets confined to his quarters. Silva's background reveals the recent death of husband and her stepchild sent to his parents (kinship) - consequently she is taking anxiety tablets.