Oskar Homolka plays a London movie-theatre owner who maintains a secret life as a paid terrorist. Homolka's wife Sylvia Sidney doesn't suspect Homolka of any wrongdoing, but she's picked up enough second-hand information about her husband's activities to arouse the interest of government agent (John Loder).
Posing as a grocer, Loder moves next door to the Homolkas, befriending Sidney and her precocious young brother Desmond Tester. Sensing that he's being watched, Homolka sends Tester out to deliver a reel of film.
The reel contains a time bomb, but Homolka is certain that the boy will deliver his package on time and will be safely away by the time the bomb explodes. Thus begins one of Hitchcock's most electrifying suspense sequences, as the unsuspecting boy is delayed en route to his destination.
Sabotage was based on Joseph Conrad's The Secret Agent; the film was retitled A Woman Alone in the US.