When swimmer Esther Robert's plane is reported to have crashed in the Mato Grosso, Buster joins the search for his friend. He finds her held captive by a renegade white man who rules a native village. The chief is delighted to have another captive, as he can sacrifice Buster to their "roarer" and keep Esther for himself. Buster defeats the green gorilla, then escapes with Esther. The natives, angered by the gorilla's death, throw the chief to the piranhas.