Yuina asks her father-in-law for help because her husband always loses his erection and she fears her own body may be the problem. Hoping for a grandchild, he examines her vagina and proposes exercises to strengthen her grip. The training works so well that her husband ejaculates almost immediately, but Yuina is left unsatisfied. She eventually turns to her father-in-law, opening the door to an inescapable affair.