England, 1947. It's an Arctic January day, one of the coldest winters on record. A man on a deserted East Anglian beach strips naked, neatly folds his clothes and wades into the icy waters. Walks to his death. His final moments are watched by Harry Tennant. The ex-copper has been assigned to observe his target by British Intelligence. The dead man is Joseph Wolff. He is a Russian Jew. He is also a professor of mathematics at Oxford University. All...