The Three Investigators - Jupiter Jones, Pete Crenshaw and Bob Andrews - are invited to Capetown, South Africa where Pete's father has been hired to build an amusement park on the mysterious Skeleton Island by the rich and arrogant Ms. Wilbur. While on a tour of the island, Ms. Wilbur is confronted by Gamba, a tribal healer, who states that the island is a sacred burial ground. She will be in great danger if she continues her plans! But Ms. Wilbur sees herself as its rightful owner, and so ignores his ominous threat...and finds herself attacked by a terrifying monster in a dark cave. Believing Gamba to be behind the attack, she has him arrested. The clever young detectives and Gamba's pretty daughter Chris embark on a desperate bid to prove Gamba innocent. With their brilliant logic, encyclopedic knowledge, and courage, the Three Investigators go on a dangerous adventure, unraveling the mystery of the Skeleton Island monster and its link to an ancient and fabulous tribal treasure.