The story is about a lustful scholar who dares to challenge the moral teachings of the Sack Monk. The monk attempts to lecture the scholar that spiritual enlightenment transcends the passions of the flesh but the scholar, who enjoys women, doesn't agree. However, the most powerful man in the town marries his daughter, played by Amy Yip, to the scholar. The daughter is a virgin and has been taught that sex is dirty. The thief tells the scholar that stealing other men's wives requires good lovemaking skills and equipment. He promises to help the scholar only if he has a horse's penis. Of course, the thief does not think it is possible, until the scholar returns one day, indeed, with a horse's penis attached as his own. Apparently, the scholar met a doctor who was able to replace anatomical parts. The doctor managed to transplant a horse's penis to replace the scholar's meager one. Armed with his new 20-inch penis, the scholar goes on a sexual rampage, not caring if he is seducing other men's wives or is nearly caught in the process. Meanwhile, the scholar's wife, after experiencing the joys of sex, becomes sexually frustrated. She tries masturbating with paintbrushes but is left unsatisfied until she has an affair with the gardener, the husband of one of the wives the scholar seduced. She becomes pregnant and runs away with the man, who out of revenge sells her to a brothel, the madam assaults her leading to a miscarriage, originally unwilling she is coerced into becoming a prostitute and comes to enjoy her new life. The scholar has become frail and sick due to too much sex. He goes to the brothel for treatment, where he is offered the top courtesan. At first, husband and wife cannot recognize each other; she looks at his penis and thinks it cannot be her husband's because his is small; he cannot recognize her because his eyesight is failing. While they are having sex, he takes a close look at her figure and nipple and recognizes her. To her dismay, he screams, shouts and calls her a disgrace. To his dismay, she runs off and hangs herself. The man, completely broken, goes back to the monk to ask for forgiveness.
Box office
In Hong Kong, the film grossed HK$18,424,224. This was enormous business considering the film was saddled with the restrictive Category III rating.
Sequels
Sex and Zen went on to spin off two sequels, Sex and Zen II and Sex and Zen III. These are not sequels in plot, as each movie illustrates a different story also based on the sex manual.
