Based on Joseph Conrad's novel, The Secret Agent, a woman learns that her husband has killed her brother in a sabotage plot.
London's lights go out, and movie customers want their money back. Carl Verloc (Oscar Homolka) tells his wife Mrs. Verlock (Sylvia Sidney) to return their money; but the lights come back. The produce-seller next door, Ted Spencer (John Loder), brings lettuce and says he saw Carl come in; but Carl denies he was gone. Spencer tells the police. Carl goes to the zoo and is instructed to put a bomb in the underground on Saturday. Spencer takes Mrs. Verlock and her young brother Stevie (Desmond Tester) to an expensive restaurant and asks about Carl. Spencer reports to the police and learns where Carl went. Carl goes to a bird-seller, who promises explosives. At home Carl writes letters. Spencer tries to listen to Carl meeting with three men, but he is caught and identified as police; the men cancel their plans. Carl tells his wife that Spencer is a police spy.
On the Mayor's show day Carl gets a package with a bird-cage, and a note that the birds will sing at 1:45. Spencer comes in and tells Mrs. Verlock that they are investigating sabotage. Carl tells Stevie to deliver the package with a film. Spencer questions Carl. Stevie stops to hear a speaker and gets his teeth brushed. He watches a parade and is allowed on a bus at 1:30. Caught in traffic at 1:45, the bomb explodes. Spencer calls in and tells Carl and Mrs. Verlock that a bus was blown up. Spencer finds a piece of the film-can Stevie had. Mrs. Verlock reads about it and faints. Carl tries to console her and makes excuses. She watches a cartoon, "Who Killed Cock Robin?" She serves dinner for Carl and stabs him with a knife. Spencer comes in to arrest Carl and tells Mrs. Verlock he loves her. She goes to find the police; but he suggests they escape together. The bird-seller's wife sends him for the bird-cage, and the police follow him. Mrs. Verlock kisses Spencer but runs off to the police. The bird-seller is trapped in the theater by the police. Mrs. Verlock says that Carl is dead. A bomb explodes, and the police tell Spencer that Carl was killed, clearing Mrs. Verlock.
This early Hitchcock thriller portrays the random violence of terrorism by not even hinting at any purpose but destruction. Carl says he won't harm people but goes ahead anyway just for the money. His coldness in making excuses for killing his wife's brother by mistake provokes her to stab him before he can grab the knife. Spencer does not trust Mrs. Verlock to a jury and is relieved she won't have to stand trial. This film was banned in Brazil because they believed it might stimulate terrorism.