Set during World War II, the book follows Alfred Vicary, a historian and friend of Winston Churchill, who joins the British intelligence service. He is assigned the job of protecting Operation Mulberry in the lead up to the invasion of Normandy in 1944. The German spy Catherina Blake, whose real name is Anna Steiner, actually is close to learning the secret, but some little failures help Alfred Vicary to reveal her true identity. So he devises and carries out his plan of Double Cross.
The basic idea of it is that after uncovering the German spy Catherina Blake, instead of capturing and imprisoning her, the British Intelligence provides her with false documents which she accepts as information she seeks. Then she sends the content of those papers through other spies to Germany, and so German Spy agencies are being deceived without having the least idea of it.
The story ends with depiction of the night Catherina tries to escape from Britain. If she could have fled she would be able to tell all she knew about British Intelligence agents and their Double Cross operation, and maybe Germans would understand that they had been deceived all the time. But Anna does not manage to escape and is killed by the fire laid down by the British martial ship. The Germans therefore remain ignorant of the secret they tried to reveal and this causes their defeat in World War II.