In Weaver's alternate historical timeline, Adolf Hitler decided to launch Operation Sea Lion (a projected German invasion of the island of Great Britain) in 1940, shortly after a more devastating version of the Dunkirk evacuation caused a shortage of British Army soldiers.

The Nazis occupy a band of territory that stretches from Portsmouth in the southwest, including communities like Tunbridge Wells, Horsham, Hastings, Pevensey, Dover, Folkestone and Gravesend.

Ben Kamen is Jewish, gay and a latent telepath, while Mary Wooler, and her son Gary, and daughter-in-law Hilda, work on covert projects for the British Army, endeavouring to discover how to dislodge the Nazi presence from the "Protectorate of Albion" in the southeast.

Ernst Keiser, a relatively kindly officer, lodges with a rural family, whose female members, Irma and Viv, collaborate intentionally with the military, while Alfie is made to serve in a forced labour unit later in the war.

However, they stumble on Ahnenerbe plans to use Kamen to alter the course of history so that either William the Conqueror lost the Battle of Hastings to Harold II in 1066, or Christopher Columbus turned east, reviving Crusader hostility against the Islamic world during the sixteenth century, instead of his discovery of America.