Marin and her younger sister, Urin, are sea creatures who chance upon an unusual discovery - a stunning silver ring lying on the ocean floor. Marin is determined to return the ring to its rightful owner, while Urin remains skeptical, recalling the fate of a turtle elder who vanished after venturing to the surface. The two sisters trek to the sky realm, where they locate the ring's owner, Kanon Miyamori, who had cast it adrift after a breakup with her beau. Despite Marin's insistence that the ring should not be discarded, Kanon opts to toss it away once more. While on the hunt for the ring, Urin grows distant from her sisters and accidentally disrupts the seal of a stone coffin. Hence emerges an evil entity known as Sedna. As Sedna rises, the missing turtle elder, Matsumoto, reappears and reveals that Marin is the Priestess of the Sea, who can join forces with the Priestess of the Sky to vanquish Sedna. During a skirmish with one of Sedna's underlings, Kanon discovers that she is the Priestess of the Sky, though she is hesitant to embrace her new role. However, with the fate of the world hanging in the balance, Kanon and Marin team up to thwart the malevolent force that threatens their homes.