An earthquake strikes Mexico, resulting in the overnight birth of a new volcano. Sent to study this phenomenon are geologists Dr. Hank Scott and Dr. Arturo Ramos. En route to the village of San Lorenzo, the two men witness a destroyed house and police car. They find a dead policeman nearby, as well as an abandoned infant.
They take the infant to San Lorenzo and give it to friends of the child's now missing parents; they are welcomed by the village's priest, Father Delgado. In addition to the disappearances of locals and the destruction of their homes, there have been wholesale slaughters of livestock and strange roaring noises in the night. The villagers believe 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, led by 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.
The volcano erupts again and the true culprits behind the disappearances and deaths are revealed as giant prehistoric scorpions. After killing a staff of telephone repairmen, the scorpions turn their attention to San Lorenzo itself, with the guns of Major Cosio's troops having no effect on them. The next morning, the scorpions have returned to their underground lair (which, in addition to the scorpions, is home to giant worms and spiders), leaving the authorities to seek the help of renowned entomologist Dr. Velasco. It is up to him, Hank, and Arturo to figure out a way to either destroy the scorpions or seal off the entrance to their cavern home, before more innocent lives are lost.
Despite collapsing the cave entrance, the giant scorpions make it to the surface and destroy a train, killing some passengers before fighting amongst themselves. In the end, one scorpion, the largest of the group and presumably the alpha scorpion, kills all of the smaller ones, making it the last scorpion alive, and it heads for Mexico City. Hank and Arturo come up with a plan to lure it to a stadium where the military is waiting with tanks and helicopters. Using a truckload of meat from a butcher shop, they manage to lure the scorpion into the stadium where the military's weapons are again proved useless against its armor. However, Hank manages to finish it off by using an electric cable attached to a spear and shooting it into its throat, which is its only vulnerable spot. After destroying several tanks and choppers, the scorpion is finally, and fatally, electrocuted.
Who do the villagers believe the culprit to be?
demon bull