The Monk: A Romance tells of the spectacular downfall of a Spanish monk. Ambroio lusts for the woman Matilda, who is disguised as a monk, but once he has had her he becomes infatuated with the innocent Antonia. This novel was the first to villainize a priest, and has all the trappings of the Gothic novel, including ruined castles, rape, incest, demonic contracts and the Spanish Inquisition.

Lewis wrote the novel in ten weeks at the age of 19.