Futako's Strange Circumstance

Ooki Futako, a third-year high school girl and avid puzzle solver, receives a package from her parents who went to explore South America. After solving one strange puzzle she wakes up the following day an additional member on her lower half. However her surprise is short-lived, as her childhood girl friend, Mune Chiisana, barges in her room.

This is the story of their relationship after this one accident.