Jack Flynn is driving down a highway at night, looking at a bracelet he has bought his daughter for her birthday. Suddenly he hits something and his vehicle crashes. The next morning, his teenage daughter Carol, is concerned her "bad-dog" father didn't come home last night. 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 orb web of an enormous tarantula, which emerges from behind some rocks to get 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 go take a look and when they are at the cave again the missing man's body is discovered drained of fluid. The spider attacks again convincing the sheriff, who orders large amounts of DDT to kill the giant spider, and appears successful. 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. As other teenagers enter the gym they begin dancing and the giant tarantula regains consciousness and the kids run out of the gym screaming while the janitor, stopping to call the sheriff, is killed.
The spider breaks out of the gym and terrorizes the town, killing a number of people before it heads back to its cave. It also pays Mrs. Kingman and her baby an unwelcome visit at their home until her husband, in his car, bangs the creature at its leg and leads it from the house. The Sheriff along with Kingman decide to use dynamite to seal the spider in, but they discover Carol and Mike are in the cave looking for the bracelet her father had bought her, which she had lost the first time in the cave. The spider chases them out onto a narrow ledge. Kingman acquires a couple of large electrodes. They run cables outside to some power lines as the tarantula is descending on a strand of web to get at the trapped teenagers. Kingman throws Mike one of the electrodes, and then they turn on the juice and electrocute the spider. The arachnid falls, impaling itself on stalagmites at the bottom of the cave.
What has he bought his daughter for her birthday?
a bracelet