Marin and her younger sister Urin are seafarers who come across something unexpected beneath the waves: a gorgeous silver ring. Despite recollections of a turtle elder who departed for the surface and never returned, the kind-hearted Marin drags a hesitant Urin along with her to the sky world, determined on returning it to its owner. They track down Kanon Miyamori, the ring's owner, and discover that she thrown the ring into the water when her lover abandoned her earlier that day.