Haruto discovers that Koyomi is the daughter of Fueki, but he keeps this to himself. Meanwhile, elementary school teacher Yoshikazu Kumagai, who comes to the Omokagedō to purchase an antique toy plane for his late son, is discovered to be a Gate when he is attacked by the Phantom Siren.

Haruto must split his attention on saving Kumagai from Siren while they try to find a toy for his son and revealing what he knows to Koyomi about her past. Meanwhile, a red crack appears on Koyomi's hand. What does this mean? Medusa and Mayu face each other again.