It's summer. The gang's on a southern island, and there's cute girls in swimsuits abound. Arata's cooking yakisoba for some reason, but these things happen. The quartermaster is repairing the school that Arata destroyed in his collapse phenomenon. The girls enjoy a mixed-gender bath, and the demon lord candidate decides on a theme--control.

The girls are running in their exercise outfits, with the boobs swaying to and fro. Arata's heart races. Everyone's having great time, and then a ghost appears in the library. Arata and the gang go to check it out, and are forcefully transported to the "Eternal Library". There they find the lost Trinity Seven!

The ultimate demon lord, Astral Trinity, shows up in the form of Arata with a grotesque new appearance. Hijiri appears as well, and the others discover that she is a high-level mage whose goal is to kill Arata and destroy the school.

Arata is set to be expelled! The only way to save himself is to be taught more about the magic archives from the Trinity Seven themselves.