Jim Wheatley disappears exploring a cave in South America. His sister Gina (Mala Powers) and her husband Dan (John Howard) go search for him. They are accompanied by Pete (Paul Richards). Pete and Gina were dating. Pete was permanently injured in an accident that was Dan's fault, and limps badly. Gina dumped Pete for Dan. Pete is angry and bitter.
When they reach the nearby village, the natives (who conveniently speak English) are hostile. There is a creepy American, Dr Ramsey, who tries to frighten them away. Jim was exploring the Cave of Death, and the natives do not want them to find it.
They find and enter the cave. Ramsey sends a native to dynamite the cave entrance, trapping them inside. The native also dies in the explosion.
Ramsey has been experimenting, creating human-fungus hybrids. The hybrids attack, but Pete drives them off with flame from the torches. They encounter a giant blob-like fungus monster.
Trying to find a way out, they discover another cave entrance beneath Ramsey's house. Ramsey reveals the fungus cave monster developed from his experiments, but is out of his control. He wants to dynamite all entrances to seal it in the cave. He sets of the explosion, trapping them all in.
Ramsey is killed by the fungus monster. Dan dies from his injuries. Somehow Pete knows the underwater river will lead them out to the sea, and will be swimmable. Pete and Gina swim out and come up on the shore.
Thanks Joseph C!