Yousuke Obata resides with his oceanographic biologist father, Tetsuo, on Pagopago in the Fiji archipelago. When a fierce storm hits the island, Yousuke sets out on his Jet Ski and unexpectedly discovers a peculiar baby dinosaur in the lagoon. He brings the creature, whom he names Coo, back to his father's lab. As Coo develops, Yousuke recognizes that the marine reptile appears to be a juvenile plesiosaur from around 65 million years ago. But Coo attracts the attention of the French secret service, whose aim is to safeguard their nuclear interests in that otherwise peaceful region of the globe.