The novel is set in London in 1886 and follows the life of Mr. Verloc, a secret agent. Verloc is also a businessman who owns a shop which sells pornographic material, contraceptives, and bric-a-brac. He lives with his wife Winnie, his mother-in-law, and his brother-in-law, Stevie. Stevie has a mental disability, possibly autism, which causes him to be very excitable; his sister, Verloc's wife, attends to him, treating him more as a son than as a brother.