On a train travelling through Bulgaria a man is shot dead, in full view of the other occupants of the compartment. This is the final act of a carefully planned operation to enable one of the passengers, Stephen Fletcher, a British spy, to take the place of the dead man - a Russian KGB agent. Fletcher is sent to join three other KGB agents, one of whom, Tonya Sklevosky, has to act the role of his wife. Together they are sent to London, where Fletcher finds himself involved in a sinister plot to cover up the work of a British traitor.