Newly married Hiroyuki has begun living with his wife Yumi's parents, but Yumi has been turning her back on him due to an interaction he had with a male colleague that he had seen a few days earlier. Hiroyuki tries to make amends by talking to her through the shower door, but when he opens it, he finds his mother-in-law Mari there! He hurriedly apologizes, but what catches Hiroyuki's eye is Mari's voluptuous, lewd breasts. He never imagined his gentle mother-in-law's overly erotic big breasts. Hiroyuki has opened a forbidden door...