"A world in which everyone is born female and some become male upon maturity (a bit like in "Simoun"). Momoka and Arisu are very close friends, and Momoka wants to become a man and marry Arisu . . . only when they end up in danger from a rapist, it is Arisu who suddenly turns male! Arisu's transformation seems to be unstable though, as "he" returns to female again when the adrenaline rush fails."

Translation from a reader.