Marin and her younger sister Urin, inhabitants of the sea, stumble upon something surprising: a beautiful silver ring lost beneath the waves. Kind-hearted Marin, determined to return it to its owner, drags a reluctant Urin along to the sky world, despite warnings about a turtle elder who left for the surface and never came back. After finding the ring's owner, Kanon Miyamori, they discover that Kanon threw it into the ocean after her boyfriend broke up with her earlier that day.
Even though Marin insists such a lovely item should not be discarded, Kanon tosses it again. While searching for the ring, Urin gets separated from the other two and accidentally breaks the seal on a stone coffin, releasing an evil entity called Sedna. Sensing Sedna's release, the previously missing turtle elder, Matsumoto, reveals himself to Kanon and her companions, declaring Marin the Priestess of the Sea. Together with the Priestess of the Sky, she possesses the power to seal Sedna away once more. And as luck would have it, during an encounter with one of Sedna's minions, Kanon discovers that she is the Priestess of the Sky. Though hesitant, Kanon and Marin decide to work together to save the world from the evil threatening it.