Li Yu begins with a chapter explaining that he's actually arguing puritan values, "making use of lechery in putting a stop to lechery"... He then introduces the Zen priest Lone Peak, who doesn't ask for alms, explicate the scriptures, or live on a mountain — and who fails to deter our hero, the Scholar Vesperus, from a life of lechery.
The Carnal Prayer Mat goes on to describe Vesperus' exploits. He marries, debauches his innocent wife, and then sets off seeking amorous adventure, leaving her in the care of his stern father-in-law. He befriends the Knave, an honorable thief, who agrees to help him find and seduce the most beautiful women. But first he has to remedy a serious deficiency, finding an adept to help him with a penis enlargement...
That problem solved, Vesperus devotes himself to six far from unwilling women. But it is his relationships with them, their relationships with one another and the social setting which provide most of the entertainment, not the raw sex.
There has to be retribution to preserve the moral balance, but in Li Yu's perverse vision this turns out to involve further debauchery. Vesperus eventually returns to Lone Pine, where Li Yu wraps up his novel with an appropriate conclusion — well, one that is appropriate given that, as he says: "This truly is a book that mocks everything!"
Li Yu includes brief authorial comments at the end of some chapters, in which he praises himself immodestly, prods fun at his critics, and otherwise entertains himself and us. Patrick Hanan offers a ten page introduction and unobtrusive footnotes which explain the literary references — mostly used for parody — and a few untranslatable puns, but The Carnal Prayer Mat really doesn't need explanation.