"Coo: Tōi Umi kara Kita Coo" is a family-friendly anime film that centers on the bond between a young boy and a prehistoric creature. The story begins when a fisherman discovers a mysterious egg on a remote island. The egg hatches, revealing a baby plesiosaur, which the fisherman names Coo. The creature is brought to the mainland, where it forms a close friendship with the fisherman’s young son, Kōichi. The two develop a deep connection as Coo grows, and their adventures together highlight themes of friendship, loyalty, and the challenges of coexistence between humans and nature.

As Coo grows larger, the challenges of keeping the creature hidden from the outside world become increasingly difficult. The family faces pressure from authorities and scientists who seek to study Coo, as well as the logistical difficulties of caring for a rapidly growing prehistoric animal. The narrative explores the tension between protecting Coo and the inevitable consequences of its existence becoming public knowledge. The story takes a dramatic turn when Coo is eventually discovered, leading to a series of events that test the resilience of the bond between Kōichi and the creature.

The film also delves into the broader implications of Coo’s existence, touching on themes of environmental preservation and humanity’s relationship with the natural world. The characters are faced with moral dilemmas, as they must decide whether to prioritize Coo’s well-being or succumb to societal pressures. The narrative builds toward an emotional climax, emphasizing the sacrifices made for the sake of friendship and the enduring impact of their extraordinary journey. Through its heartfelt storytelling, the film presents a poignant exploration of love, loss, and the enduring connection between humans and the natural world.