Scotland Yard detectives attempt to solve a spate of safe robberies across England beginning with clues found at the latest burglary in London. The film is notable for using a police procedural style made popular by Ealing in their 1950 film The Blue Lamp. It is known in the US as The Third Key.

At the start of World War II, Cmdr. Ericson is assigned to convoy escort HMS Compass Rose with inexperienced officers and men just out of training. The winter seas make life miserable enough, but the men must also harden themselves to rescuing survivors of U-Boat attacks, while seldom able to strike back. Traumatic events afloat and ashore create a warm bond between the skipper and his first officer.