Rin, Shima and Izumo have been sent on a mission at the shore, only to find themselves working at Ocean House? There, they meet a boy named Yohei, and learn of a monster known as the "Demon of the Deep." Yohei has lost his father to this demon, and is planning for revenge. Worried that he'll take on the demon, Rin and the others vow to keep an eye on him. Just then, the sea undergoes a bizarre change!