In the depths of the sea is a family of three. Astrya, a young syrena, lived in isolation deep in the ocean's trenches with her mother and her childhood friend Yona, a water dragon. While her mother and Yona would leave, Astrya would stay in the cave, only leaving to hunt for food. One day, a merman, Mizuchi, brightening everything around him, finds his way into the cave, holding her injured mother. The two must travel away from home to keep everyone...