During a vacation on a secluded island, Shiina Tamai encounters a celestial starfish-like being while exploring an ancient shrine underwater. Saved from drowning by this enigmatic creature, whom she names Hoshimaru, Shiina forms a deep bond with it as it displays magical abilities. Their whimsical adventures take a perilous turn when they are embroiled in a cosmic conflict between warring alien groups riding colossal beasts known as Shadow Dragons in the skies, with the destiny of Earth at stake.