Jill Young has been caring for the orphaned gorilla Joe since her mother's death. Due to a rare genetic anomaly, Joe has grown to a height of 15 feet. Jill and Joe live in the mountains of Africa and live a peaceful life until a group of poachers led by Andrei Strasser arrives. Jill's mother is killed while protecting the gorillas, and Jill is left alone to care for Joe.
Years later, Gregg O'Hara, a zoologist from California, arrives to visit Jill and Joe. He immediately becomes fascinated by Joe and convinces Jill that he would be better off living in a wildlife refuge, where he could be protected from danger. Jill reluctantly agrees and Joe is transported to the U.S.
However, Strasser has followed them to the U.S. and is determined to capture Joe for himself. Strasser believes that Joe's enormous size will make him a valuable commodity on the black market. He hires a group of henchmen to help him capture Joe, and chaos ensues as Joe attempts to protect himself and his new human friends.
With the help of O'Hara and a team of rescuers, Jill and Joe are able to fend off Strasser and his henchmen. Joe is eventually returned to the mountains of Africa, where he can live out his life in peace.