It is 1952 and the Second World War has not happened. Europe is, instead, presided over by a Third Reich embroiled in an unwinnable war with Russia, and held back from civil war only by a terminally ill Adolf Hitler.
This is the world the English historical novelist CJ Sansom presents us with in Dominion, his latest outing, in which he follows in the footsteps of Len Deighton's SS-GB and Robert Harris's Fatherland.
Sansom's occasionally flawed story is exciting, rich in historical detail and populated with finely drawn characters who are plucked from their mundane existence and thrown into a world that is about to be turned upside down.
The book opens with Lord Halifax, one of the British government's arch-appeasers of Hitler, becoming prime minister in 1940 (instead of Winston Churchill) after the debacle of Dunkirk. Rather than fight on, Halifax sues for peace and Britain becomes, in the words of one of Sansom's characters, "a drab conformist German satellite".
The country descends into a totalitarian state, complete with censored press, a banned opposition and a new generation of elaborately-quiffed youths called "jive boys" who like nothing more than loud music and "a good ruck" on a Saturday night. Churchill, who makes a cameo appearance, leads the resistance.
Against this background emerges David Fitzgerald, a disillusioned civil servant, spying part-time for Churchill's underground government, while his wife becomes quietly distraught, believing he is having an extramarital affair.
Dominion turns when Dr Muncaster, Fitzgerald's old university friend, surfaces unexpectedly. Muncaster has a secret, one that requires Fitzgerald to deliver him into the hands of the Americans before the Gestapo gets there first.
And it is this secret, and how Muncaster comes by it, that is the book's first problem: can one impart enough detailed scientific information to materially revitalise a nuclear weapons research programme in the space of a five-minute street brawl? And can the recipient of that information remember enough to enable the Third Reich to finally build an atom bomb? The whole imperative for the novel rather depends on these answers.
Another plot problem is the British people. Even a passing acquaintance with the history of the British Isles will reveal a cross-grained, bloody-minded folk rarely willing to do as they are told. Their track record in telling authority to back off dates back to Magna Carta in 1215 - when the English people held a sword to King John's throat while he signed it - and little has changed since.
In other words, would the British people have allowed such a dystopia to grow up around them without a fight? How you respond to that question will shape your opinion of this novel.