Mama Katsu is about a guy who is visited by his brother. Both have a small meeting to celebrate the move of Kyoko, a newly married mature woman. Now that her man is away on a business trip, the protagonist can't hide her desire to make him her bitch.

He and Momoka, Kyoko's close friend, are having an affair at a love hotel. Late at night, thanks to all the sex Momoka had for hours, she can't take it anymore and she turns into a woman who can't think about anything other than sex.

Yoshiya is a private teacher who is teaching Kako, a young student. In addition to giving her academic lessons, the teacher sexually educates her. Her voice in stasis was heard by her mother Fumika, who sees everything that was going on. Instead of doing something to save her daughter, she becomes very horny and gives in to her lustful instincts even though she is married.

Kyouko, a mother whose husband is away on a trip, agreed to go on a date with her friend Touka and Touka’s lover, not knowing that he is her younger brother. During the date, Kyouko felt uncomfortable, especially when Touka left her alone with Yoshiya. When trying to go back home, Yoshiya took her to a room and showed her his erect penis, to Kyouko’s surprise.

Kyouko, a charming mother, offers herself to Yoshiya, who desires her madly, under the condition that everything must end afterward. Though she tries to act cold and hopes he will eventually tire of her, her plan fails against Yoshiya’s insatiable desire.

Far from being the crying boy who once followed her around, he now embraces her from behind with clear sexual intent. Worried but unable to resist, Kyouko secretly gives in to his demands.