the spy in black (1939)

aka U-boat 29.
"If I am to be shot it is as an officer."
1917 and a German u-boat captain (Conrad Veidt) is sent to the Orkney Isles to aid in the blockade of Britain and help sink some of the British fleet. To do so he is ordered to report to the local schoolmistress (Valerie Hobson), who is a German spy. She has a means to help him in his mission.
Powell and Prestburger's first collaboration is a spy story with a twist. Much of it follows the U-boat captain's story in a dispassionate way, making him less of a villain then one would suppose, but then the film was made a year before the second world war started.