Tetsurou Obata is a marine biologist specializing in oceanographic biology, residing with his son Yusuke on Pago Pago Island in the Fiji archipelago. Their home doubles as his laboratory for scientific research. After Yusuke discovers an unusual creature in the lagoon following a storm, Tetsurou identifies it as a baby plesiosaur—a species extinct for 65 million years. He supports his son in caring for the creature, named Coo, providing scientific oversight and shelter within his lab as it grows.

When French military and intelligence operatives led by Captain Rooslean and Colonel Nolver target Coo to conceal nuclear testing activities, Tetsurou allies with Catherine "Cathy" Ono, a Japanese-Canadian environmental activist connected to the Coast Guard and Green Earth. Together they protect Coo from exploitation. Tetsurou's scientific background guides his actions throughout, balancing empirical observation with ethical responsibility toward the rare creature. Escalating confrontations with the French operatives force him to navigate both research challenges and protective measures for Coo's survival.