Kosuke discovers Medusa's true identity as the school-aged girl Misa and attacks her, only for Rinko to arrest him. Going on the magician's hunch that she's after another schoolgirl as a Gate, Haruto and Shunpei attend the school to try to thwart Medusa's plans.

When it is discovered that Mayu is the twin sister of Misa, the girl who has become Medusa, Mayu makes it her mission to seek out the one part of her sister that is still alive within Medusa, even though Haruto believes it is better to save Takako first.