Jack Flynn is driving down a highway at night looking at a bracelet he has bought his daughter for her birthday, when he hits something and his vehicle crashes. The next morning, his teenage daughter Carol is concerned that her ne'er-do-well father did not come home the night before. She convinces her boyfriend Mike to assist in a search for her father. They find his crashed truck and the bracelet, but not his body. Thinking he crawled into a nearby cave, they investigate. In the cave, they fall onto the gigantic web of an enormous tarantula, which emerges from behind some rocks to attack them. They manage to escape and make it back to town.
Carol and Mike have a hard time convincing the sheriff about the giant spider, but with the help of their science teacher, Mr. Kingman, they return to the cave and find the missing man's body, drained of fluids. The spider attacks again, convincing the sheriff, who orders large amounts of DDT to kill the giant spider. The apparently lifeless body of the spider is taken back to town to the high school gym, where Kingman wants to study it. A group of teenagers uses the gym to practice rock and roll numbers they are going to play for a school dance. The music awakens the giant tarantula and it crashes through the wall of the gym. The janitor, stopping to call the sheriff, is killed.
The spider terrorizes the town, killing a number of people before it heads back to its cave. The sheriff and Kingman use dynamite to seal the spider in, but then discover Carol and Mike had gone into the cave to retrieve the bracelet her father had bought her. Kingman acquires a couple of large electrodes from the power company and runs cables to some power lines as the tarantula is descending on a strand of web to get at the trapped teenagers. Kingman and Mike use the electrodes to electrocute the spider, which falls, impaling itself on stalagmites at the bottom of the cave.
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