Coo, a baby plesiosaur from a prehistoric marine reptile species existing approximately 65 million years ago, originates in an unspecified distant ocean region. After a severe storm strands this infant creature in a lagoon near the Fiji archipelago, Yusuke Obata—a boy residing on Pagopago Island—discovers it.

Yusuke brings Coo to a laboratory overseen by his father, marine biologist Tetsuo Obata. The Obatas decide to nurture the creature, naming it Coo. As Coo matures under their care, its plesiosaur identity emerges through distinct physical traits and behaviors. During this period, Coo forges a strong bond with Yusuke and Tetsuo, frequently interacting within their domestic and scientific spaces. Their household also includes a dog named Contseau.

Coo's presence draws intense external scrutiny, particularly from French military and intelligence groups active in the South Pacific. These entities view Coo as a threat to undisclosed nuclear testing operations and interests in the region. Their mission shifts toward capturing or containing the creature to suppress public knowledge of its existence and its habitat, which coincides with nuclear test zones. This pursuit escalates into direct attempts to seize Coo, triggering confrontations involving Yusuke, Tetsuo, and their allies.

The narrative chronicles Coo's journey from discovery and adaptation to human society to the external perils that endanger it. Coo's existence directly challenges human environmental impacts, though definitive outcomes for the character are unspecified.