Aiko is a school girl that all her friends are menstruating and not her. She goes home to her her room and reads up on menstruation and punches herself in the uterus in frustration.
This seems to "awaken" Ma-chan, her 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.