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.