Deep within a mountain ravine lays a nature-rich countryside town. And at the bottom of its beautiful mountain stream, "another world" spreads out as if reflecting its water surface.

The protagonist, Mio, is a camera geek who currently lives in Tokyo. Every summer, she returns alone to the town that she was born and raised in. In the summer of her second year of high school, she was playing in the river with her old friends as usual when she accidentally discovered a secret of her childhood friend, Kouhei's. This eventually brought about the development of a major incident that would shake both them and the "world".

This is a youthful suspense fantasy adorned with glaring sunlight and cool, clear water.