At school, Aiko discovers that all the girls in her circle of friends have already had their first menstruation save her. After going home to her room, she reads up on menstruation and in frustration punches herself in the uterus. This seems to "awaken" Ma-chan, Aiko's anthropomorphic vagina, who begins coughing up blood. Ma-chan introduces herself and says that she's been with her ever since birth and from now on she's going to assist in her journey into adulthood.
The next day at school Aiko tells her friend that she's now one of the gang.
Note: Aiko no Maa-chan, was cancelled by Comic Zenon from fears of its publisher that the manga could be considered an harmful publication under the Tokyo Metropolitan Ordinance Regarding the Healthy Development of Youths. She decided to publish her manga through a successful crowdfunding campaign on Fundiy. Thus Aiko no Maa-chan will be published as one volume in 2015.