The "Bagmen Incidents", a series of murders. The perpetrator is the rumored girl clad in a sailor uniform whose face is concealed by a Gas Mask dubbed "Mask-san". One day, by a coincidence, that same Mask-san was seen by the young boy Watta. Furthermore, that face belongs to, of all people, his classmate…!? With the unprecedented heroine, Gas Mask Girl unfolds with suspense in battle and on campus. The curtain rises magnificently!!