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.