Robert "Rack" Hansen (William Shatner), a veterinarian in the rural town of Verde Valley, Arizona, receives an urgent call from a local farmer, Walter Colby (Woody Strode). Colby is upset because his prize calf has become sick for no apparent reason, and brings the animal to Hansen's laboratory. Hansen examines the calf, which dies shortly afterward. Hansen tells Colby he cannot explain what made the animal so ill so quickly, but takes samples of the calf's blood to a university lab in Flagstaff.A few days later, Diane Ashley (Tiffany Bolling), an arachnologist, arrives looking for Hansen. Ashley tells Hansen that the calf was killed by a massive dose of spider venom, which Hansen greets with skepticism. Undaunted, Ashley tells him the problem is serious and that she wishes to examine the animal's carcass and the area where it became sick. Hansen escorts Ashley to Colby's farm. Moments after they arrive, Colby's wife, Birch (Altovise Davis), discovers their dog is dead. Ashley performs a quick chemical test on the dog's carcass and concludes that like the calf, it died from a massive injection of spider venom. Hansen is incredulous, until Colby states that he recently found a massive "spider hill" on a back section of his farmland. He takes Hansen and Ashley to the hill, which is covered with tarantulas. Ashley theorizes that the tarantulas are converging together due to the heavy use of pesticides, which are eradicating their natural food supply. In order to survive, the spiders are joining forces to attack and eat larger animals.Hansen and Ashley return to the Colby farm. As the scientists and the Colbys are walking past a barn, a bull stampedes out; it is being attacked by tarantulas. Ashley notes that the spiders likely will not be afraid to attack people either. Colby douses the spider hill with gasoline and lights it on fire, seemingly destroying the spider menace. However, many of the spiders escape out of a tunnel. Colby is attacked in his truck the next day, sending it over the side of a hill and killing him. Hansen happens upon the accident scene and helps the sheriff, Gene Smith (David McLean), examine the wreckage. Colby's body is found encased in a cocoon of spider webs. Meanwhile, Ashley is notified by her colleagues that a sample of venom from one of the spiders is five times more toxic than normal. Hansen is then told by the sheriff that several more spider hills have been located on Colby's property.Hansen, Ashley and the sheriff examine the hills along with the mayor of Camp Verde (Roy Engel), who orders the sheriff to spray the hills and the surrounding countryside with a pesticide. Ashley protests, arguing that pesticide use is what caused the problem to begin with and that the town would be better off using birds and rats (tarantulas' enemies in nature) to eradicate them. The mayor dismisses the idea, fearing that having a large number of spiders and rats all over the countryside will scare away patrons of the annual county fair. A crop duster is enlisted to spray the pesticide. Once the pilot is airborne, he is attacked by tarantulas, and crashes the plane before he can disperse the spray.The spiders begin their assault on the local residents, killing Birch and Hansen's sister-in-law, Terri (Marcy Lafferty). Hansen arrives at their home and rescues Terri's daughter, Linda from the spiders. Hansen and Ashley take Linda to the Washburn Lodge. They consult with the sheriff, who tells them that the spiders are everywhere and Camp Verde is cut off from the outside world. Smith drives into town, while Hansen and the other survivors at the lodge plan to load up an RV and escape. However, the spiders trap them in the lodge, and they barricade themselves inside. Meanwhile, Smith arrives in Camp Verde and finds the town under siege by the spiders. Smith tries to escape, but is killed when another car crashes into a support post under the town's water tower, causing it to fall on his vehicle.Back at the lodge, the power goes out, and Hansen is forced to venture into the lodge's basement to change a blown fuse. He succeeds, but is besieged by spiders who break through one of the basement windows, by using their combined weight. He makes it upstairs just in time to be saved by Ashley.The film concludes the next day, with the survivors rigging up a radio receiver and listening for news of the attacks. To their surprise, the radio broadcast doesn't mention the attacks, indicating that the outside world is oblivious to what has happened. Hansen pries off the boards from one of the lodge's windows, and discovers that the entire building is encased in a giant web cocoon. The camera pulls back, and all of Camp Verde is encased in cocoons as well.
Output: Question: Why does the bull stampedes out?, Answer: it is being attacked by tarantulas.