It is the Taisho era, and Koharu, a young girl who has lost her parents, is taken in by relatives. Treated coldly by her stepparents, she decides to become a sacrifice to the terrifying water god, rumored to be both a dragon and a giant snake, in place of her beloved younger sister. "It may be a tiny life, but if it can save my adorable younger sister..." At the moment she is attacked by the giant snake calling itself the water god, someone in the shape of a beautiful human appears...! A tale of pure love and interspecies marriage between a water god at the mercy of fate and his sacrificial bride!