German U-Boat commander Captain Hardt (Conrad Veidt) is sent on a mission to break through the British naval blockade and to rendez-vous with a German agent in the Orkney Islands. Hardt's mission is to sink ships of the British Royal Navy docked in the harbour. His contact is the beautiful Frau Tiel (Valerie Hobson) a German agent posing as a school teacher. With the help of Tiel he engages the treacherous British naval officer Lt. Ashington (Sebastian Shaw) who will guide him to the harbour. All is not as it seems in this thrilling spy story as Hardt is double crossed by his accomplices. Ever the proud patriot Hardt refuses to wear anything but his black German Navy uniform. If he is to die, it will be with honour as a sailor and not as a spy.