The movie opens with an exploding volcano in Mexico. A narrator describes the latest volcanic eruption. Title and Credits follow. A Jeep with a trailer is being driven over the volcanic debris by an American Geologist, Dr. Hank Scott (Richard Denning). He is accompanied by Dr. Arturo Ramos (Carlos Rivas) a Mexican Geologist. The area is still smoldering. They encounter a telephone crew working to restore phone service. They ask directions to San Lorenzo and if anyone has made it through. They are told a police car went through earlier and hasn't returned. Arturo and Hank stop driving when they hear a strange roar. The Jeep needs water and they see a farm house ahead. They call out for anyone home at the farm house. Arturo looks for a well and some water for the Jeep as Hank goes into the house calling out. Arturo calls Hank back outside after he finds a damaged and abandoned police car. The police radio calls for Sgt. Vega, but he is nowhere to be found. Hank starts to call on the radio, but thinks Arturo should do the talking. He hands him the microphone to explain the wreckage to the dispatcher. They hear what sounds like a rattlesnake in the house. They go in to investigate and find a baby still alive. The house is a shambles. Hank carries the baby boy back to the Jeep. They find the dead policeman amid the wreckage outside. They report that and drive to San Lorenzo.They arrive in San Lorenzo amid a crush of terrified peasants. The local Priest, Father Delgado (Pedro Galvan), comes to greet the two scientists. Hank gives the baby to the priest. A local woman identifies him as Manuel Tiburcio. His parents are missing and presumed dead. She takes the baby. Father Delgado explains, over a meal, that the deaths began the first Sunday after the eruption of the volcano. Hank asks about the superstition of the huge "demon bull." Delgado explains that, "The demon bull is a symbol of evil among many ancient civilizations." All the men of the Miraflores Ranch abandoned the place two days ago. They brought the demon bull story with them. Hank and Arturo spend the night then leave the next morning. Major Cosio (Arturo Martinez) asks Hank and Arturo not to being their expedition. They set off on their journey, but tell Cosio they intend to return to San Lorenzo that evening. Once on the road, they stop to get their bearings and plan their approach. Hank spots a woman on a white horse. He gives Arturo the binoculars to look, but by the time he gets them in focus the rider has fallen off. All he sees is a horse, which surprises Hank because he saw a rider. And a beautiful woman at that. They drive off to find her. Teresa Alvarez (Mara Corday) is all right, just a little embarrassed. She is the owner of the Miraflores Ranch and has been thrown by her horse, Lucero, before. She tells the two scientists that Lucero will find his way back to the ranch on his own. Arturo goes to retrieve her saddle and sees a piece of obsidian. Teresa asks for a ride to San Lorenzo. She needs ranch hands to help with her cattle. She also confirms the two geologists will make Miraflores their base camp. When she arrives in San Lorenzo she talks her ranch hands into returning to the ranch. As Arturo and Hank enter the church, they are summoned to see the Major and Dr. de la Cruz (Pascual Pena). They are witness to the policeman's autopsy. De la Cruz suspects an organic poison but he is unable to identify the exact one with his primitive field lab.Arturo, Hank, and Teresa drive to Miraflores. Her ranch hands follow on horseback. She invites them into the house. Arturo comments, "Not only comfortable, but cool too." She calls to her housekeeper Florentina (Fanny Schiller) and Juanito (Mario Navarro) to assist her guests in getting settled. Hank and Teresa engage in small talk. It is obvious Hank is romantically interested in his hostess. Arturo calls them over to examine the rock they picked up earlier. Inside the obsidian is a trapped scorpion. When Arturo breaks it open a live scorpion escapes. Hank puts it in a jar. Before Hank and Teresa can go outside, the telephone rings. She excuses herself to answer it. The lineman explains her phone line was the last to be fixed and he is testing the line. A man on the ground turns around and is confronted by a huge scorpion that roars and drools. It grabs the man and carries him under a bridge into a dry river basin, or arroyo. The man still on the pole notices the commotion and calls to his colleague, Gaetano. Gaetano gets in the phone truck, but the scorpion flips it over killing the man. He chases the lineman back up the pole, picks him off, and kills him. Arturo, Hank, and Teresa run to the arroyo bridge. The dog, Pancho, runs off. Juanito chases after the dog, and sees the giant scorpion. He flees in another direction as Hank, Arturo and some ranch hands fire their rifles at the creature.Another volcanic eruption damages the church. Refugees scatter as a giant scorpion attacks. The cattle stampede and the occupants of Miraflores Ranchero evacuate to the city of San Lorenzo. They encounter Dr. Velazco (Carlos Muzquiz) who just arrived amid the commotion. Dr. Velazco tells them he was sent to be in charge of emergency arrangements. San Lorenzo is to be evacuated. Velazco identified the poison as scorpion venom. He proposes poison gas as a weapon against this newly discovered species--Scorpionida Rex.The ranch hands, and the army search for the lair of the scorpions. Hank, Arturo, Teresa, and Dr. Velazco drive along and spot dead cattle. A man on a horse finds the opening around the volcano, but falls in off his horse. Hank and Arturo decide to go down into the crevasse. When they go to their trailer to get some climbing equipment they discover a stowaway, Juanito. Hank and Arturo dress in protective clothes; Hank has a camera. Hank and Teresa kiss. The two geologists get into a metal cage and are lowered down into the cave. Midway down they discover a large scorpion on a ledge. Hank takes its picture and they continue their descent. At the bottom they find themselves in a huge cavern. They exit the cage and begin to explore. They are unaware that Juanito has joined them on their exploration of the cavern. They spot and photograph a huge inchworm and find the remains of the man who fell in, Mendoza. A nest full of scorpions approaches. Juanito exits the cage to look around on his own. A scorpion attacks the giant inchworm. Then a larger scorpion attacks a smaller one, killing it with its stinger. Juanito pulls at a trap door and is chased by a giant tick. He hides in a cave as Hank and Arturo dispatch it with their rifles. The three run back to the safety of the cage, but the scorpion arrives first and destroys it. They watch helplessly as the creature snaps the cage from the line. Arturo hangs on to the line as it is pulled up. He is safe on the surface. They make a loop on the cable and send it down for Hank and Juanito. They are pulled to the surface. Velazco orders the opening closed with explosives.Velazco is back in Mexico City. He calls Hank in San Lorenzo and orders him and Arturo to Mexico City for their report. Velazco arranges an airplane with room for three. Hank wants Teresa along. The three arrive in Mexico City and are met by Victor Esteban. He takes them to the University to meet with Dr. Velazco. Hank asks for an explanation. Recent photographs of the San Lorenzo area showed renewed activity. There is an underground system of caves and caverns that might be used by the scorpions. One arm of the system is twenty miles from Mexico City.Hank and Teresa go to a night club for a drink. They discuss dinner plans. They have dinner while a train from Monterrey to Mexico City is attacked by the scorpions and derailed. A swarm of them attack and kill the passengers. A larger one, the Black Scorpion, arrives and kills some of the smaller ones. Hank and Teresa hear the report on the radio in their rented car after dinner. The Black Scorpion is headed for Mexico City. The city is ordered evacuated. The military enters the city as does the giant arachnid. The plan is to lure the scorpion into an arena then shoot it with a projectile attached to an insulated wire then electrocute the beast with 600,000 volts. They bait the arena center with tons of steer carcasses. When the scorpion arrives it crawls over the stands and on to the floor of the stadium. Tanks and helicopters attack the creature. The first shot of the projectile misses, but the power was applied anyway. A soldier retrieves the projectile and is electrocuted. Hank and Arturo reload the gun with the projectile. Meanwhile the scorpion is making short work out of the tanks and helicopter firing on it. The second shot hits its mark. The power is applied and the scorpion dies. Hank and Teresa depart the arena, arm in arm. The rest stare at the dead scorpion. We close with a zoom out to a long shot of the dead scorpion.