Yousuke Obata is living with his father, Tetsuo, a marine biologist, on Pagopago Island in the Fiji archipelago. A storm sweeps through, and while riding his Jet Ski, Yousuke discovers a strange baby dinosaur in the lagoon. He brings the creature back to his dad's lab and gives it the name Coo.
As Coo matures, it starts to look like a prehistoric marine reptile—specifically, a young plesiosaur from about 65 million years ago. But Coo's presence catches the unwanted notice of the French secret service, whose job is to protect France's nuclear activities in this ordinarily tranquil region.