Episode 4

S1 E4: A police investigation collides with the security services and the Royal Navy in an atmospheric mystery from the makers of "Line of Duty," in which a fishing trawler's inexplicable disappearance leads investigators below the waves into a shadowy world of intrigue. When a crew member is found dead aboard the Trident nuclear submarine HMS Vigil, the Scottish police begin an inquiry. The Vigil must remain on patrol in order to maintain the UK's nuclear deterrent, so Detective Amy Silva is forced to join the crew on board to conduct her investigation. Meanwhile, Detective Kirsten Longacre 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.