Umi Monogatari

Marin and her younger sister Urin, two girls from the sea, discover an unusual treasure on the ocean floor: a gleaming silver ring. Eager to return it, gentle-hearted Marin convinces the reluctant Urin to accompany her to the surface—despite warnings tied to a turtle elder who once left for the sky world and never came back. Their search leads them to Kanon Miyamori, a human girl who admits she threw the ring away after being dumped earlier that day.

When Kanon discards the ring again, the three set out to retrieve it, but the search takes a dangerous turn as Urin becomes separated and accidentally breaks the seal on a stone coffin, unleashing the malevolent Sedna. The missing turtle elder, Matsumoto, appears and declares Marin the Priestess of the Sea, destined to join forces with the Priestess of the Sky to seal Sedna once more. During a confrontation with one of Sedna’s servants, Kanon learns she is that very priestess, and despite her uncertainty, she and Marin choose to stand together against the threat rising from the deep.