After his high school graduation, Yuuki Wakura still hasn't gotten over his younger sister's disappearance five years earlier during a Mato incident, when portals to a demon-filled dimension began opening across Japan. He accidentally frees Kyouka Uzen, commander of the Anti-Demon Corps' Seventh Unit, from a magical seal, binding himself to her as a "slave" whose life force sustains her fighting power against the invading Shuuki. Season 3 continues the pair's partnership as Wakura is drawn deeper into the Seventh Unit's campaign against the monsters pouring out of Mato.