Sayo is a young girl living in a small village by a magical lake. Every night, the lake reflects not one, but two moons. Legend has it that beneath the lake's waters lies an entire world, and whenever something important happens in that world, one of the moons changes color. One day, Sayo meets a strange young man named Kaé, who turns out to be a messenger from the underwater kingdom. His world is on the verge of destruction, and only Sayo can help him restore balance between the two worlds. Together, they journey into the underwater kingdom, where time flows differently and they confront ancient myths and their own fears.