A search party goes into the jungles of Mexico to find a missing test pilot. What they find is a gigantic, one-eyed, mutated monster who became as such through a dose of radioactivity.

After Susan Winters' fiancé, Bruce Burton, is reported missing in a plane crash in Mexico, she organizes a search party to fly into a mountainous area to look for him. In addition to Susan, the group includes Martin Melville, a stock speculator who believes there is uranium in the mountains and has partially financed the trip, Russ Bradford, a bacteriologist friend of Burton and hired pilot Lee Brand. After they land their light plane in Guayjorm, Susan is denied permission to proceed by the local governor, who is very suspicious of the group's motives for being there. The governor orders them to leave and assigns an officer to accompany them out of Mexico. At the airstrip, however, Melville knocks out the officer and they take off for the area where Bruce's small plane was known to have crashed. Melville becomes very excited when a meter he is carrying indicates large uranium deposits in the mountains. Suddenly the plane hits air pockets and Melville panics, knocks out Lee and takes over control of the plane. From the seat behind Melville, Russ tries to regain control as they hurtle toward the ground, but Lee recovers in time to land the plane safely. When Melville confirms that the rocks are rich in uranium, he wants to leave immediately to file a claim, but the others remind him of the true purpose of the search. The next day as Susan and Russ set off on foot to the site of Bruce's crash, Russ tells her that he is in love with her and is tired of competing with a man who is probably dead. After coming upon a giant bird attacking a giant rat, they return to the plane where Susan takes possession of the ignition keys as she fears Melville may bribe Lee to take off without them. When Russ and the others witness a fight between a giant iguana and a giant lizard, Melville becomes even more determined to leave, but Russ tells him they will all leave together once their mission is accomplished. In order to determine how the animals in the area have grown to be so enormous, Russ removes skin tissue from the dying iguana and, after examining it in a microscope, determines that there is no limit to the potential size of the animals as their cells, stimulated by radioactivity, are continuously subdividing. Russ then warns the others that they are in danger of being similarly affected and they should all leave as soon as possible. The next morning, Susan sets off alone and finds a wing section of Bruce's plane, then is abducted. Her screams lead the others to a cave where they find more wreckage, Bruce's flight jacket and watch and the hysterical Susan. Suddenly, a giant, twenty-five foot tall, male human, with a hideously deformed face and only one eye, blocks the exit from the cave and roars threateningly at the group. That night, when the Cyclops attempts to communicate with the group, Susan tells him her name, but Melville picks up a rifle and shoots at him. The Cyclops retaliates by killing Melville, then lifts Susan up and places her on a rock outside the cave. When a giant snake threatens Susan, the Cyclops rescues her, and in the chaos Russ and Lee escape from the cave. Suddenly realizing that the giant might once have been her fiancé, Susan becomes emotionally distraught. After the Cyclops falls asleep, Russ, Lee and Susan run back to the plane and are about to take off when the Cyclops reappears and they are forced to flee. Russ then creates a burning stake, which he launches into the Cyclops' eye, mortally wounding him. Susan, Lee and Russ start the plane and manage to take off as the Cyclops once again approaches. Russ comforts Susan as she looks down on what was once her fiancé.