A man (Oscar Homolka) who runs a cinema is part of a group that is plotting an explosion in London. He is being investigated by an undercover policeman (John Loder) working in the greengrocers next door. As the policeman investigates the plot starts to unravel and his exploits are unknown to his wife (Sylvia Sydney) and her son (Desmond Tester). This is one of Alfred Hitchcock's films that he made in England, before moving to America. The film concentrates on the characters and especially the relationship between the policeman, the man and his wife and her child. This is a good example of Alfred Hitcock's early work. The film is based on the novel The Secret Agent by Joseph Conrad.