Living on the island of Pagopago in the Fiji archipelago with his father, oceanographic biology expert Tetsuo, young Obata Yousuke's life takes a thrilling turn when a storm hits the island. Exploring the lagoon on his Jet Ski, he stumbles upon a peculiar baby dinosaur, whom he affectionately names Coo. As Coo grows, revealing itself as a prehistoric marine reptile, the secret of its existence attracts the attention of the French secret service, drawing unexpected complications to their serene surroundings.