[1] It takes place near Scapa Flow in Orkney during the First World War.

[2] German U-boat officer Lieutenant Conrad Von Belke travels to Scotland as part of a plot to gain intelligence for an attack on the Royal Navy fleet as it leaves its anchor.

He encounters a local minister and a British naval officer prepared to betray his country.

Too late, Von Belke realises he has been part of an elaborate British deception operation to lure U-boats out to sink them.

In 1939 it was made into the British film of the same title directed by Michael Powell and starring Conrad Veidt, Valerie Hobson and Sebastian Shaw.