Why would a journalist who was an ardent socialist and an anti-Nazi during the waning years of the Weimar Republic decide to go to work for the Gestapo abroad? Hans Wesemann, a veteran of World War I and a successful journalist, fled his native Germany in 1933 after writing a number of anti-Nazi articles. Once in Britain, he found life difficult and dull, and thus, for a number of reasons, agreed to furnish the German Embassy in London with information about other refugees. Inevitably, Wesemann became ensnared in his own treachery and suffered the consequences.

During the volatile and...