Harry Compton, the orphan son of a Russian, loses his job at the Bank of England as a result of courting Penelope Lightfeather during his lunch hour. Despising the capitalist system, he reveals to East End restaurateur Bagda, a friend of his late father, that he intends to go to the Soviet Union. Bagda, a secret Soviet agent, instead recruits him to steal the secret Admiralty file "Avalanche," dealing with the mobilization of NATO forces in time of war, from the safe of security chief Sir Reginald Dumfrey. Unbeknownst to the conspirators, false plans have been fabricated for the purpose of tracing the chief Soviet agents. To facilitate the theft, Sir Reginald gives the file to his gossip-loving wife, Lady Barbara, and arranges a reception to attract the spies. Harry discovers that Penelope works as a couturière at the home of Lady Barbara, and he enlists her aid in gaining entrance to the reception. The file is alternately lost and found as a series of misadventures threaten to compromise the entire Admiralty project. In the end, however, the spy chiefs, Bagda included, are destroyed in a shootout, and Harry is united with Penelope, who, as it turns out, is a British agent.