An earthquake strikes Mexico, resulting in the overnight birth of a new volcano. Geologists Dr. Hank Scott and Dr. Arturo Ramos are dispatched to study this crisis at the village of San Lorenzo, the two men finding a destroyed house and a totaled police car en route. They find a dead policeman nearby, and an abandoned and seemingly orphaned infant. They take the infant to San Lorenzo and give it to friends of the child's missing parents, while being welcomed by the village's priest, Father Delgado. Delgado reveals the property damage is caused by something that is slaughtering the livestock, the villagers believing the culprit to be a demon bull and have been pestering Delgado for divine assistance.
Undaunted, Hank and Arturo begin their geological survey as members of the Mexican army under Major Cosio arrive in San Lorenzo to begin disaster-relief efforts. Hank meets and falls in love with local rancher Teresa Alvarez, and makes friends with a young boy named Juanito. When the volcano erupts again, the culprits behind the disappearances and deaths are revealed as giant prehistoric scorpions. After killing a crew of telephone repairmen, the scorpions attack San Lorenzo with the Mexican military unable to harm them. The next morning, the scorpions have returned to their underground lair (which is also home to giant worms and spiders), leaving the authorities to seek the help of renowned entomologist Dr. Velasco. Velasco enlists Hank and Arturo to figure out means of destroying the scorpions or seal off the entrance to the cavern.
Despite collapsing the cave entrance, the giant scorpions took refuge in the caves before resurfacing days later to attack a train. Some of the passengers are killed as in-fighting among the scorpions resolves with the largest of them killing the others before heading for Mexico City. Hank and Arturo come up with a plan to lure it to a stadium with a truckload of meat, with the military distracting it long enough for Hank to kill it by shooting an electric cable attached to a spear into its throat, which is its only vulnerable spot. After destroying several tanks and helicopters, the surviving fighters detonate the electric charge, finally slaying the last scorpion.
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