In a peaceful archipelago world filled with water parks and seaside attractions, two riders and lifeguards, Rhea and Nico, usually spend their days rescuing drowning victims and assisting tourists. One day, they are assigned a simple mission: investigate the source of a strange roar echoing across a remote sea and report their findings to the chief of the Coast Guard. However, the routine assignment takes an unexpected turn when they encounter a gigantic sea serpent that should only exist in the deepest parts of the ocean. Ignoring orders to merely observe, Rhea decides to provoke the creature and lure it back into the open sea, believing that risking herself to protect others is what defines a true rider. Reluctantly, Nico follows his reckless partner, hoping her bold plan won't turn into a disaster.