Umi Monogatari - S01

Marin and her younger sister Urin are seafolk who come across something very strange: a beautiful silver ring lost beneath the waves. Kind-hearted Marin, wanting 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 learn that Kanon threw it into the sea after her boyfriend dumped her earlier that day. Marin insists that such a lovely thing shouldn't be thrown away, but Kanon tosses it again. While they search for the ring, Urin gets separated from the other two and accidentally breaks the seal on a stone coffin, releasing an evil being called Sedna. Sensing Sedna's release, the turtle elder Matsumoto, missing until now, appears before Kanon and her companions and names Marin the Priestess of the Sea. Together with the Priestess of the Sky, she has the power to seal Sedna away again. As luck would have it, during a run-in with one of Sedna's minions, Kanon discovers that she is the Priestess of the Sky. Though Kanon hesitates, she and Marin decide to work together to save the world from the evil threatening it.