It is the year 1983. Tired of living with her promiscuous and disorganized roommate, Samantha Hughes decides to rent a small apartment of her own. Although her landlady (Dee Wallace) graciously agrees to waive the deposit, Samantha finds herself needing to scrape together $300 for the first month's rent. In her search for quick cash, she responds to a babysitting advertisement. The man behind the ad informs her that he and his wife require her services for the night while they attend a party to witness a lunar eclipse, where the moon will fall entirely into the earth's shadow at midnight. Despite his strange demeanor during the call, Samantha's financial desperation pushes her to accept the job.
Her best friend, Megan](/actor/greta-gerwig), drives her to the secluded house where the babysitting job will take place. Upon arrival, Mr. Ulman, the man who posted the ad, reveals that Samantha won't be babysitting children, but rather caring for his elderly mother-in-law. Initially hesitant, Samantha is persuaded by Mr. Ulman's offer of $400, even in light of Megan's protests. After dropping Samantha off, Megan makes a stop at a nearby cemetery to smoke a cigarette. Here, a young man named Victor (AJ Bowen) unexpectedly appears at her window, offering a light. When she denies being the babysitter, Victor fatally shoots her in the face.
Samantha meets Mrs. Ulman, who emerges from the basement, searching for her furs. In a peculiar exchange, Mrs. Ulman remarks on how attractive Samantha must be to young men. The Ulmans provide Samantha with half of her pay along with additional cash to order a pizza. Mr. Ulman assures her that his mother-in-law won't be a bother and mentions that they can be reached at a phone number they leave behind.
Samantha attempts to do some homework but finds the house unsettling. As she dances with her Walkman placed snugly over her ears, she accidentally shatters a vase. While cleaning up the mess, she discovers a closet filled with luxurious fur coats and a box full of photographs featuring another family, not the Ulmans, posing in front of the house and a vehicle she noticed earlier. When the pizza arrives, it's Victor who delivers it, before casually wandering around the property, surveilling her.
Doubting the quality of the pizza, Samantha takes a few bites before discarding the remainder. Unbeknownst to her, the family from the photos lies gruesomely slaughtered in an upstairs bedroom, their bodies arranged in a blood-soaked pentagram. Hearing strange sounds, she armed herself with a knife and ventures to investigate the attic. As the clock strikes midnight, all lights in the house fail, and she sees a hand ajar the attic door. Soon after, she loses consciousness due to the drugs hidden within the pizza.
When she regains her senses, she's bound and gagged on a slab in the basement, encircled by sinister satanic symbols. The Ulmans, which include their son Victor, along with a dwarfish demon, enter the scene. The demon draws a pentagram upon Samantha's abdomen and attempts to force her to drink from an ominous animal skull. In a surge of desperation, she manages to free herself and rushes upstairs. There, she sees Megan's lifeless body in the kitchen, seizing a knife as Victor follows her, shooting her in the shoulder. In a fierce altercation, Samantha slits Victor's throat, killing him, but is left with excruciating abdominal pain and terrifying flashes of demonic faces.
Cornered by Mrs. Ulman in a bedroom, she learns that it is too late to change the horrifying plan. Distracted by an assumed message from Satan due to the eclipse, Samantha seizes the opportunity to stab Mrs. Ulman in the back. Armed with Victor's gun, she attempts to escape, pursued by Mr. Ulman, who is wounded from her earlier escape. He tries to rationalize her fear, foretelling that Satan's arrival is imminent when the eclipse ends. In a moment of despair, believing there’s no way out, Samantha takes her own life by shooting herself in the head.
Meanwhile, local news anchors express confusion over why the moon seemed to eclipse faster than anticipated. As Samantha lies in a comatose state in a hospital, a nurse assures that both she and her unborn child are expected to recover fully.
