Yuuki Wakura has spent years looking for the older sister who vanished into Mato, the demon-filled dimension whose portals opened across Japan. When he stumbles into Mato himself and Shuuki corner him, Kyouka Uzen, commander of the Mato Defense Force's Seventh Unit, pulls him out by making him her "slave". Her power draws out the hidden strength of whoever she enslaves, and she owes him a reward for the work he does. Season 3 keeps the pair in the Seventh Unit's fight against the creatures pouring out of Mato.