The film is based on 17th century Chinese erotic novel by Li Yu best known as The Carnal Prayer Mat. Mio is a struggling writer who writes a pornographic book entitled Flesh Futon which becomes a surprise best seller. With his newfound wealth, Mio begins indulging in the nightlife, excusing his activities to his wife by saying it is for research. Mio's reputation comes crashing down when a prostitute reports that his penis is like a "guppy". More bad luck follows. His book is banned, and his wife puts him on a short leash. At the height of his misfortune, his house is robbed. Mio meets the robber who turns out to be the Japanese folk-hero Nezumi Kozō. The two become friends, and Kozō introduces Mio to a doctor who can perform penis-enlargement surgery. With his new equipment, Kozō and Mio become frequenters of the red-light district, until the real Nezumi Kozō appears, and evil luck returns to Mio.