Itogami Island's tourist industry flourishes as the season of the annual Halloween Fest draws near. Kojou reunites with his childhood friend Tokoyogi Yuuma for the first time in four years, but Yukina and Asagi are not pleased to see the intimate friendship she shares with him. Meanwhile, when the Meyer Sisters suddenly appear at the Keystone Gate, the "witch of the void," Natsuki, finds herself in danger...

Still stuck in Yuuma's body after the mind switch, Kojou rushes to see Yukina and the other girls. At first they're unsure whether to believe him, but after an explanation, Kojou and Yukina try to locate Yuuma in Kojou's body as they draw closer to unraveling the mystery of the witch of the void.

Yuuma's goal was to use the Fourth Progenitor's magic power to free her mother from the prison barrier. In accordance with her plans, the prison barrier appears above the ocean's surface near Itogami Island.