Millennia ago, Mother — an ancient water deity — was imprisoned beneath the earth when light first came into the world. Her mate, Otoshigo, retreated to the ocean's depths. For generations since, Mother has dispatched human avatars to the surface to prepare for her return, but each one has failed and never returned to the Underworld.
In 1976, the power goes out on Yamijima Island when an underwater cable is severed. A four-year-old boy named Shu Mikami discovers a young woman washed up on shore — a woman named Kanae who bears a striking resemblance to his dead mother. They become inseparable, and Kanae moves in with Shu and his father.
The islanders distrust her, unnerved by her aversion to sunlight. They are not wrong to be wary: Kanae is one of Mother's avatars, tasked with guiding Shu into opening the seven sealed gates on Yamijima Island that keep Mother imprisoned. Before she can complete her mission, Tsuneo Ohta — leader of the local fishermen — decides she must be killed.