Kannazuki Sara, a young woman rooted in a tropical coastal community, becomes entwined with the quest for the mythical Blue Butterfly Fish. A pivotal encounter from her past—a meeting with Ushio Hayama a decade before the story’s events—reverberates through time when the two reunite during his deep-sea diving expedition. This reconnection ignites a profound metamorphosis in Ushio, unlocking his latent power to control water and shaping his trajectory into competitive swimming and diving.

Drawing from regional myths, Sara guides Ushio and his companions, her insights into the Blue Butterfly Fish’s lore anchoring their journey. While her knowledge steers the group’s pursuit through vivid tropical landscapes and oceanic depths, the narrative refrains from definitive resolution, preserving the legend’s enigmatic allure.

Confined to a single-episode OVA, Sara’s arc intertwines Ushio’s history with his newfound abilities, serving as both a catalyst for his transformation and a conduit for the group’s immersion into the setting’s mysteries. Her presence bridges memory and destiny, yet the story concludes without unraveling her personal future or the full scope of her past, leaving her role etched into the fleeting currents of the tale.