This is the Kitahara household. Natsumi has allowed her son Haruyuki to have sex with her vagina, but instead has allowed him to have sex with her anus, saying, "That's the only thing you can't do." Natsumi has come to the point where she can climax through anal sex. Meanwhile, at the Matsumoto household. Marina has allowed her son Seiji to have anal sex with his mother, but instead has allowed him to have sex with her vagina, saying, "That's the only thing you can't do." Seiji has dreamed of Itsuka day having anal sex with Marina... One day, Haruyuki and Seiji decide to take a certain action...