Ushio Hayama wields the rare gift of underwater respiration and liquid manipulation, traits vital to his free-diving pursuits in tropical island waters. His childhood bond with Sara leads them to her relatives’ home, where they organize dives to uncover the fabled blue butterfly fish rumored to dwell nearby. During these expeditions, they cross paths with Aruuyu, a cryptic island girl whose matching aquatic powers suggest an undisclosed kinship to Ushio’s own.

As Ushio interacts with Aruuyu and her sister, their groups unite to hunt the elusive fish. Tensions flare when Aruuyu, envious of Ushio’s closeness to Sara, entices Sara into the Dragon’s Mouth—a hazardous network of submerged caves. With surging tides endangering Sara, Ushio’s mastery over water and steady composure allow him to brave the labyrinthine tunnels, securing her rescue. Their emergence from the depths coincides with a symbolic convergence between Ushio and the blue butterfly fish, implying an otherworldly bond.

Ushio’s origins and the root of his powers stay shrouded, his poise in crises underscoring adaptability as his defining trait. The ocean’s secrets whisper through his actions, though their full significance lingers unexplored. His ties to Sara and Aruuyu weave a tapestry of rivalry and alliance, anchored in their shared mission yet confined to the expedition’s fleeting timeline.