Set in the Scottish Orkney Islands during the First World War, this is the story of three German spies plotting to sink the British fleet. U-Boat Captain Hardt (Conrad Veidt) makes contact with his beautiful co-conspirator (Valerie Hobson). He falls in love with her, but she is already having an affair with the third spy in their group, Royal Navy traitor Lieutenant Ashington (Sebastian Shaw). The tensions in the group affect their operation as a unit.

Ken Annakin directs this World War Two drama based on a novel by Nevil Shute. Michael Denison stars as Rick, a young RAF pilot patrolling the English Channel for U-Boats. When he sinks what he believes to be a German submarine, it later turns out to be British and he is charged with neglect. In order to clear his name and prove his worth, he volunteers to lead a dangerous mission. Meanwhile, his girlfriend Mona (Patricia Plunkett) fights to prove that Rick is innocent of the charges made against him.