September 1942. The RMS Laconia travels the Atlantic Ocean carrying Italian prisoners of war, British troops, and civilian passengers. While Junior Third Officer Thomas Mortimer befriends the enigmatic Hilda Smith, beneath the sea, a German U-boat looks for targets.

A tentative truce forms between the sailors and the survivors. Mortimer is taken aboard the U-boat, and Hilda reveals her secret. Hartenstein's distress call has surprising consequences both for the rescue operation and the future conduct of the war.