After the excitement of being promoted to ExWire, Rin is soon disgusted with his load of homework. Then, Yukio is summoned for an urgent mission! The familiar of the Knights of the True Cross has gone berserk, and Yukio has to deal with the crisis.

Rin, Shima and Izumo have been sent on a mission, only to end up working instead. They meet a boy named Yohei and learn of the "Demon of the Deep." Yohei lost his father to this monster, and wants revenge. Worried, Rin and the others watch over him.