James Rutherford has organized an expedition to the jungles of Central America to find his missing son, Bob, and his guide, Simpson. Professor Dangerfield intercepts the party, bringing with him Simpson, whose jungle experience has made him a raving maniac. They go ashore and decide to spend a night at a Mayan temple. After Irene, Bob's fiancée, disappears, they come across a gigantic ape chained to the floor, and Captain Rhodes, commander of the yacht, is abducted by a strange monster with great hairy talons. Messages are found warning the party to leave. Sewald, an explorer, is mysteriously killed by an arrow. Simpson's reason returns, and he saves the party, revealing that the demented hermit, whom he has just killed, and who formerly occupied the ruins, murdered Bob two months before.