This is the story of one of the most enduring conspiracy theories in British politics, an intrigue that still has resonance almost a century later: the Zinoviev Letter of 1924. Almost certainly a forgery, no original has ever been traced, and even if genuine it was probably Soviet 'fake news'. Despite this, the Letter still haunts British politics nearly a century after it was written; it was the subject of major Whitehall investigations in the 1960s and 1990s, and cropped up in the media as recently as during the Referendum campaign and the 2017 general election. The Letter, encouraging...