It is 1983. Frustrated with living in the dorms with her oversexed and messy roommate, Samantha Hughes (Jocelin Donahue) rents a small apartment. Although the landlady (Dee Wallace) agrees not to charge her a deposit, Samantha still needs to come up with $300 for the first month's rent. She answers an advertisement for a babysitter. The man who placed the ad tells her that he and his wife need her that night while they attend a party to watch the lunar eclipse; the moon will be entirely in the earth's shadow at midnight. Although he behaves oddly on the phone, Samantha's desperation leads her to accept the offer.
Samantha's best friend Megan (Greta Gerwig) drives her to the job, which is at a secluded house in the woods. There, Mr. Ulman (Tom Noonan), the man who placed the ad, reveals that Samantha will not be babysitting but taking care of his elderly mother in law. Samantha tries to back out but when Mr. Ulman offers her $400 she agrees to stay over Megan's objections. Megan leaves. On the drive home, she stops in a nearby cemetary to have a smoke. When her lighter malfunctions, a young man, Victor (AJ Bowen) suddenly appears in her window and offers her a light. He then asks her if she's the babysitter. When she says no, Victor shoots her in the face, killing her.
Samantha meets Mrs. Ulman (Mary Woronov) when she emerges from the basement. Mrs. Ulman explains she was looking for her furs then makes Samantha uncomfortable by commenting on how desirable she must be to young boys. The Ulmans give Samantha half of the babysitting fee plus some money to order a pizza. Mr. Ulman assures Samantha that the mother in law will be no trouble and will probably keep to herself in her upstairs room. They also leave a phone number where they can be reached if anything goes wrong.
Samantha tries to do homework but finds the house creepy. She orders a pizza and, while dancing around the house listening to her Walkman, accidentally breaks a vase. While cleaning it up, she sees that the closet is full of fur coats. She also finds a box full of photos of a family other than the Ulmans, posing in front of the house and a car that Samantha saw in the driveway earlier. Victor delivers the pizza to Samantha, then wanders around the property to keep an eye on her.
Unbeknownst to Samantha, the family she saw in the photos lies slaughtered in an upstairs bedroom, arranged as parts of a bloody pentagram. Samantha hears strange noises and, armed with a knife, goes to explore the attic. At midnight, all of the lights in the house blow out. Samantha sees a hand open the attic door, and she passes out.
When she awakes, she is gagged and tied to a slab in the basement, surrounded by satanic symbols. The Ulmans -- including their son, Victor -- and a dwarfish demon enter. The demon draws a pentagram on Samantha's belly, then tries to force her to drink blood from a bizarre animal skull. Samantha breaks free of her bonds and rushes upstairs. Seeing Megan's body in the kitchen, Samantha grabs a knife. Victor chases her and shoots her in the shoulder but she slits his throat, killing him. Samantha is wracked with abdominal pain and flashes of demonic faces. Mrs. Ulman corners her in a bedroom and tells her that it is too late for Samantha to alter the plan. Mrs. Ulman is distracted when she believes the eclipse is sending her a message from Satan, and Samantha stabs her in the back. Grabbing Victor's gun, Samantha flees from the house, pursued by Mr. Ulman, who was wounded when Samantha escaped from the basement. He tries to calm her down and tells her that Satan will arrive as soon as the eclipse is over. Believing it to be the only solution, Samantha shoots herself in the head.
Local newscasters talk about how astronomers are confused that the moon seemed to move through the eclipse more quickly than it should have. Samantha lays comatose in a hospital bed but she and her baby are expected to make a full recovery.