Harold Guppy moves into the Beasley household as a lodger. Before long Mrs. Beasley falls for him and eventually ends up in his bed. Her 13-year old daughter Joyce is aware of what is happening and threatens to tell Mr. Beasley unless she is also allowed share the bed with Harold and "Mum". "Mum" seems to think there is no harm in this, as long as Joyce doesn't take part in the physical activities. Harold gets caught up in a web of deceit and blackmail and each time he tries to break free of the grasp of "Mum" she threatens to tell the police that he has been abusing Joyce. The film continues much in this line, and it examines the effect of the relationship on both Harold and Joyce and how they are both driven to extreme action because of the influence of Mrs. Beasley.