"I never thought it would turn out like this..." My girlfriend's younger sister, Rena, comes to visit the house where I live with my girlfriend. The next day, she leaves for a three-day business trip, and Rena returns soon after, pleading with me to stay with her because she's lonely. It's a very hot day and we're both covered in sweat, but Rena, who has an abnormally strong libido, clings to me and seduces me. Unable to resist, I finally make a mistake, and we begin a forbidden physical relationship, consumed by her sexual desire and devouring each other desperately from morning until night.