A delirious archaeologist stumbles back into his group's camp after exploring a nearby cave, leaving his partner behind. The expedition, led by Prof. John Fielding [Leslie Daniels], watches as the man slides toward madness and ends up in the hospital, a stark reminder that something in the cave may be more dangerous than anyone guessed. Intrigued by this alarming turn, the rest of the team resolves to press deeper into the cave’s mysteries.
Inside the dark chamber, they uncover a deep pool and a towering statue of Caltiki, the vengeful Mayan goddess who once presided over human sacrifices. In a bid to recover artifacts, one of the expedition members, Bob [Daniele Vargas], descends into the pool. At the bottom, he discovers a trove of skeletons adorned with gold jewelry and drags up as much treasure as his cables will allow. Though warned not to descend again, he insists that the riches could make them all wealthy, and the others reluctantly consent to send him down a second time. As he pulls upward, the cable’s movements grow erratic, and when he is hauled back to the surface, his flesh has withered away, leaving only a decayed mass over his skull and bones beneath his face mask.
From the pool, a shapeless creature erupts, attempting to digest anyone within reach. A brief struggle ensues as a team member is caught by the mass’s grasp but is rescued in time. The blob slowly slips from the cave’s threshold and begins to crawl toward the outside world. Nearby, a tanker truck loaded with gasoline becomes a crucial point in the desperate effort to stop the threat. One brave scientist guns the tanker directly into the advancing mass, and the resulting explosion sets the creature ablaze, destroying it in a fiery burst just outside the cave.
The survivors return to Mexico City to treat the wounded and to contain the mystery. The recovered flesh from the arm of the injured man is taken back to the expedition’s circle, and surgeons carefully remove the fragment, revealing it to be nothing more than a few moist scraps connected to bone. Researchers then determine that the creature is a unicellular bacterium that grows rapidly in the presence of radiation. A comet emitting radiation and set to cross Earth’s path only once every 850 years looms on the near horizon, intensifying the urgency. As the remaining piece of the blob is stored at the home of expedition member Dr. John Fielding [John Merivale], the scientists fear what could happen if it continues to reproduce unchecked.
Fielding fights to convince the Mexican government to act before the reproducing blobs spiral out of control, but he is arrested and eventually escapes. A colleague finally persuades authorities to issue a warning, and the situation escalates quickly. The government must mobilize a regiment of soldiers armed with flamethrowers and jeeps to confront the growing danger at Fielding’s home. Upon arriving, they find the amorphous mass has multiplied and overrun the house and grounds. Fielding’s wife, Ellen Fielding [Didi Sullivan], and their child hide on a second-floor window ledge, narrowly escaping the hungry reach of the blob. Fielding arrives just in time to save them, as the soldiers unleash torrents of fire to reduce the threat to a smoldering ruin. The scene closes with the family reunited amid the ashes of the once-mysterious creature, while the future of the remaining blob remains uncertain.
