A man driving along a lonely back road at night is suddenly startled by what he sees, and is promptly killed by something that crashes through his windshield. The next day, in the nearby town of River Falls, teenagers Carol Flynn (June Kenney) and Mike Simpson (Gene Persson) decide to go looking for her father, who didn't get home last night. They find his wrecked truck and enter a nearby cave to begin searching for him. There they find his blood-covered hat and other signs of human remains and, as they go deeper inside, suddenly get trapped in a huge web -- then they spot its maker, a spider the size of a small house. They manage to escape and alert the county sheriff (Gene Roth), who doesn't take them seriously but does heed the warning of Mr. Kingman (Ed Kemmer), the science teacher at the local high school, to bring a pest-control crew along with his deputies, and a tanker loaded with DDT. They encounter the creature, and, after losing one of their men, dispatch it with the insecticide. Kingman persuades the sheriff to bring the carcass into town so that he can arrange to have it studied, leaving it in storage at the high school recreation room, for lack of anywhere bigger to keep it. As it turns out, the creature isn't dead, just stunned. As the local rock & roll band rehearses, the giant spider comes to bloodthirsty consciousness, breaking out of the building and ravaging the town. Bullets won't hurt it -- as Kingman says, you could punch holes in it all day without hitting a vital spot -- and the town is soon cut off when the telephone lines are knocked down.