Oskar Homolka is a London cinema-house manager who uses his wife and her teenage brother as cover, for he is also working with a gang of foreign saboteurs.

Scotland Yard assigns John Loder to work next door to the cinema to observe Homolka, but he finds himself spending more time observing his wife (Sylvia Sidney).

When a bomb destroys a city bus, killing her younger brother, she uncovers her husband's "secret" and decides to put an end to his murdering schemes.

Public opinion turned against Hitchcock for allowing a child to be killed in such a grisly manner. He later conceded the 'bus bomb' scene was wrong for it built up the audience's suspense but then didn't relieve it.

Film was banned from exhibition in Brazil because it upset public order.

The cartoon shown at the Bijou, "Who Killed Cock Robin?" was by Walt Disney.

Released in the U.S. under the title, "The Woman Alone."