Based on Joseph Conrad's novel The Secret Agent, the film was produced in the years immediately preceding World War II, and the unnamed hostile power behind the bombings is assumed to be Nazi Germany. At the time of its release, Europe was already preparing for war, and espionage was rampant, particularly between the European powers. The film can therefore be interpreted as a warning to civilians to be aware of foreigners, who threatened the safety of the country.
Hitchcock took considerable liberties with the novel, transforming the highly political anarchists and socialists into foreign agents without any obvious political leanings. The tobacconist's shop is tranformed into a movie theater (with the films being shown echoing the story), and the policeman investigating the case is cast as an undercover officer posing as a greengrocer. To critics, however, the most troubling change was in the character of Stevie, Mrs Verloc's young brother, who is portrayed as a simpleton, with few of the visionary attributes of his literary counterparts. Stevie's death is a climactic moment in the plot, providing insight into Hitchcock's views about how the innocent suffer through random acts of violence. When a critic condemned Stevie's death as brutal and unnecessary, however, Hitchcock refused to defend his position and said that he regretted including it in the film--although with this he remained faithful to the novel.
The fact that the film was set in a movie theater allowed Hitchcock to reference the plot with contemporary films and storylines. Perhaps the most famous of these is the final film sequence, an animated short produced by Walt Disney.