While some of the exact characters and events may be fictional, the book is based on very real events- the attempt by the Allies to use British intelligence to cover up the true plans for D-Day. The deception plan was called Operation Fortitude, and Double Cross also played a role. Specifically, the book has a backdrop (a subset of Fortitude referred to as Fortitude South).
The story is about the crucial events that decided the outcome of World War II.
German Intelligence tries to learn the time and place of invasion into France by Allied forces. The future of the war depends on this information, so British Security Agencies and MI5 in particular try to conceal the truth from Germans by giving them false information. All this is accomplished by the MI5 official Alfred Vicary, a former professor of History in one of London Universities. 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 Catherine 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.