An earthquake in central Mexico triggers a seismic upheaval that births a new volcano overnight, drawing the attention of two scientists who travel to the village of San Lorenzo to study the crisis. Dr. Hank Scott Richard Denning and Dr. Arturo Ramos Carlos Rivas arrive to find a destroyed house, a wrecked police car, and the corpse of a nearby officer. They rescue an abandoned infant and deliver the child to friends of the missing parents, all while being welcomed by the village priest, Father Delgado Pedro Galván. Delgado explains that livestock have been slaughtered by something unseen, and the locals have begun to fear a demon bull, begging for divine intervention as the crisis deepens.
Undaunted, the two scientists begin a careful geological survey as the Mexican army, under Major Cosio Arturo Martínez, arrives to coordinate disaster relief. A spark of romance flickers when Hank Richard Denning meets Teresa Alvarez Mara Corday, a local rancher, and he befriends a curious young boy named Juanito Mario Navarro. When the volcano erupts again, the mysterious disappearances and deaths are traced to a far more terrifying culprits: giant prehistoric scorpions that threaten the town and its people. The creatures slaughter a crew of telephone repairmen and then turn their fury on San Lorenzo, prompting a brutal struggle the Mexican military finds it cannot win.
The scorpions retreat to an underground lair, leaving authorities to seek the help of renowned entomologist Dr. Velasco Carlos Múzquiz. Velasco dispatches Hank and Arturo to scout the cavern and determine whether poison gas could reach the insects’ vast subterranean network. Along the way they contend with dangerous beasts—giant worms and swarming spiders—and discover a behavior that could be pivotal: the scorpions kill each other by striking a small white spot on their throats. This discovery shapes their strategy, but the cavern’s enormity makes gas impractical to deploy effectively; Velasco concludes that sealing the entrance with dynamite is the only viable option, though Hank worries about potential alternate entrances.
With the cavern supposedly sealed, the threat seems contained—until the scorpions resurface days later to attack a train, spreading panic and chaos as some passengers perish. In escalating in-fighting, the largest scorpion finally diverts its rage toward Mexico City. Hank and Arturo devise a bold plan: lure the giant creature to a stadium with a truckload of meat while the military creates a diversion long enough to kill it. The plan hinges on a charged spear wired to an electric cable, aimed at the scorpion’s vulnerable white throat spot. The first attempt misses, and a fatal mistake follows when the power remains on and the gunman is electrocuted while reloading. Undeterred, Hank Richard Denning takes the challenge again, and with precision finally succeeds in slaying the last of the terrible giants, bringing an uneasy calm back to a country stunned by a geological nightmare.
