The thriller focuses on an investigation into the disappearance of a fishing trawler and a death on-board the titular submarine. As you'd expect from
Line of Duty production company World Productions, this all ties into a conspiracy that goes deep (pun not intended).
Looking at the trailer, it seems like Suranne Jones' investigator is being set up by her superiors — is one of them H? — while the submarine itself looks like it will be quite the claustrophobia and paranoia-inducing environment.
Plus we're just happy to see Martin Compston again now that
Line of Duty series 6 has come to an end.