Sigmund Rosenblum is on a mission in Russia. His train is stopped and he is arrested, but he escapes and travels to London. There, he compromises Margaret Thomas, the young wife of a rich clergyman. Her husband dies, Rosenblum marries her, takes her maiden name (becoming Sidney Reilly) and is posted to the Far East.
Reilly is in France, searching for Margaret, his wandering wife, and an Australian oil tycoon, and also competing with the Rothschilds to buy oil concessions in the Middle East. While Reilly is in Paris, he meets his long-lost half-sister, Anna, and we learn more about their beginnings in a prominent family in Odessa. But Anna kills herself, amid strong intimations of incest.
Reilly and Grammaticoff arrive in Moscow with a million dollars in cash. Reilly plans to overthrow the Bolsheviks, who are in the process of making peace with Germany, and to replace them with a new Russian government led by him. Once in power, Reilly plans to renew the war against Germany. Meanwhile, the British are planning an invasion of Russia.
Reilly makes careful plans to take control of Moscow, but his coup misfires when Lenin brings forward a meeting of the Congress. Then most of Reilly's friends are arrested by the secret police after an anarchist makes a bungled assassination attempt. Lenin survives being shot, and Reilly flees Moscow.
It's New York, in the year 1924, and Reilly persuades Henry Ford to put up the money for an invasion of Russia. But Reilly is betrayed by Eugenie, his secretary, and kills her. In New York, Reilly finds Nadia Massino - and he makes a hit with Pepita. Meanwhile, the Zinoviev Letter is published in The Times of London (25 October, 1924), a few days before the British general election. It seems to be a letter from Zinoviev, head of the USSR's Comintern, to communists in Britain, and is a huge embarrassment for the British Labour Party. The Conservatives win the election and Anglo-Soviet treaties are abandoned. So is the Zinoviev letter forged or genuine? Reilly is a key player.