Robert Harris, Len Deighton and many others have posed the question "What if the Nazis won the Second World War?", but it's an idea that's yet to run out of steam.

In CJ Sansom's alternate universe, the timeline branches off at a meeting in 1940 in which Churchill is passed over for the post of Prime Minister in favour of Lord Halifax. So horrified is Halifax by the losses in Norway and France that he surrenders, paving the way for Britain to become a puppet Nazi state.

Dominion picks up the thread again in 1952. Lord Beaverbrook is Prime Minister, with Oswald Mosley his Home Secretary. Britain has retained its empire, in return for giving the Nazis carte blanche in Europe, while Germany is still throwing men into the meat grinder of the Russian front. In hiding, Winston Churchill is directing a resistance campaign.

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Among those who mourn the loss of self-determination and old-fashioned British values is David Fitzgerald, a civil servant whose Irishness conceals a secret Jewish heritage. Having been recruited into a resistance cell, he's assigned to go to a lunatic asylum and try to prise information out of an old university friend, Frank Muncaster.

Muncaster is neurotic, scared of his own shadow. His brother, who works in weapons research in the United States, drunkenly let slip a military secret with such far-reaching potential consequences that Frank freaked out and ended up being committed. Unfortunately, the Germans have got wind of this too. Everyone wants what's in Frank Muncaster's head, but they have to go about it in ways that won't attract attention.

Sansom's crime-writing background is perfect for a suspenseful political thriller like this, much as his historical novels give him a head start when it comes to creating an alternative 1950s. As well as knowing how to build tension, he understands the importance of not making it easy to sort out the heroes from the villains.

Dominion is a variation on an old theme that finds there's still much vitality left in it. Nevertheless, enjoyment of the book is badly soured by Sansom's appended historical note, which equates the SNP with the kind of nationalism found in 1930s Germany, a breathtakingly bold assertion that is at odds with the nuance and subtlety contained in the novel proper.