Warning: Wikipedia contains spoilers.
Upon leaving the store, they are offered directions to go to the area where Dr. Satans legend grew. They pick up a hitchhiker, who leads them to a house in the area. There, they meet the hitchhiker's family. This hitchhiker turned out to be one of a group of five local cheerleaders who had been reported missing a few days before. At first, the family seemed charming and hospitable, but they actually turned out to be a family of psychopat murderers, and the four other cheerleaders who had disappeared turned out to have been kidnapped by the family.
Both couples are also kidnapped, tortured, terrorized and enslaved by the family. Despite their frequent attempts at getting out, either by trying to run away from the place or pleading with the family, every minute turns more and more painful for the couples.
Meanwhile, local cops investigating the disappearances arrive at the house, and they are shot to death by the family. The four cheerleaders kidnapped are also murdered.
After one of the couples dies, the girl in the other couple tries to run away from the house, only to get into a room where she discovers that Dr. Satan is still alive, and about to open her boyfriend up on a medical operating table set up specially for him, while the man was still alive. After witnessing her boyfriend's death, she escapes and is able to make it to the road leading to town.
She hitchhikes, but the man she gets in the car with turns out to be the convenience store owner, and she ends up at Dr. Satans table herself.