As daughter of the former demon lord, Mio's foster parents were slain by powerful demons after her life. Still seeking to slay her, Basara reveals himself as a member of the hero clan--an enemy of the demons--and vows to protect Mio as a member of his family.

Basara goes on a date with Yuki. Unaccustomed to the shopping center, when the two of them are overwhelmed by a clerk's sales talk, Mio stops following to rescue them.

Ten days pass since the battle with Takashi and others. Although the battle ended with reconciliation, Yuki returned to the hero's village. ...or she was supposed to, but when Basara returns home, he finds Yuki in an apron!

Basara and Yuki begin the ceremony to form their pact. Clad in a bustier and garter belt, Yuki was raring to go for it, but she refused to forge the pact normally, and suffered the same succubus curse as Mio.

The day of the sports festival is finally here. Though Basara remains cautious of the mysterious enemy who manipulates people with magic, he happily enjoys the sports festival with Mio and Yuki as the three of them compete in the four-legged-race and enjoy their youth getting entangled and sweaty with each other. Meanwhile, Kurumi, who was placed in charge of guarding the school, discovers a suspicious individual. At almost the same time, a giant twister forms on the sports field. Basara fights to protect the students... but an unexpected individual appears!