【SYNOPSIS】Treasure Ship: Latitudes of Lust begins with a scene of a man and woman in the Edo Period making love. The two try to imitate sex positions from shunga, or traditional erotic paintings, but in doing so the man strains his back and cannot continue. As the senryu poem muses, "A foolish couple, imitating shunga, get it all twisted." Some shunga sex positions, when attempted in real life, can lead to a very painful climax. But perhaps it is human nature to try foolish experiments in sex.
In the present, less than a year has passed since Kazuo married Chiharu. He is perfectly happy. One night, Kazuo hears Chiharu mumble the words "treasure ship" in her sleep. Takarabune, she says—the name of the mythical ship that carried the Seven Lucky Gods. Kazuo starts to wonder… What kind of dream did Chiharu have? Was she dreaming that she had become one of those seven gods, perhaps the goddess Benzaiten?
One Sunday, Kazuo takes Chiharu to visit his uncle Kenji and aunt Toshiko. After dinner, Kenji secretly shows Kazuo a rare book. The book is an erotic photo album illustrating the shijuhatte or "48 Secret Techniques"—a list of sex positions based on the 48 traditional sumo throws. Kazuo finds one image in the album that especially draws his interest. The picture is labeled "Treasure Ship."
Later that night, Kazuo dreams of Chiharu and an unknown man performing that exact Treasure Ship position. Kazuo wakes up startled and embarrassed by the dream he just had. "Chiharu is so pure and innocent. How foolish of me to dream of something like that," he thinks. "But I would sure like to try the Treasure Ship with her…"
Several days later, Toshiko meets Kazuo in private and lets him in on a secret that he never would have imagined. She tells Kazuo that his wife Chiharu and his uncle Kenji have been having an affair for several years. Aunt Toshiko suggests, "Let's get revenge against them!" But how would they do that?
Reeling from shock, Kazuo doesn't know how to respond. He hates his cheating wife, but at the same time he still loves her dearly. In his confusion, Kazuo hears a voice in his head: "I bet Chihiro is doing the Treasure Ship with uncle Kenji. But I've never even tried it!" At that moment, Kazuo devises a plan for revenge.
But what could that revenge be? And how will it change the relationship between these four adults?