The Harmon family moves from Boston to Los Angeles to escape their troubled past, and make their home in a house with a past far more troubled.
Two red-headed twin boys are looking to vandalize a house with baseball bats. Before entering, they encounter Addy outside, who warns them not to enter the house. She tells them that if they do, they will die. They brush off this warning and proceed to enter the house. Entering the basement, they find a number of specimen jars, including animal remains and human fetuses, as well as bloody surgical tools. Complaining of the smell, Bryan starts walking back upstairs until he realizes his brother, Troy, has gone silent. He walks back downstairs to find Troy gasping for air, his throat slit by the Infantata.
Vivien Harmon is undergoing an examination by her OB/GYN, Dr. Day. He is trying to convince her to take hormones to increase her sex drive. She refuses, her explanation being that she may just be depressed. He asks her what she is afraid of, and it becomes apparent she recently had a stillborn baby. She returns home, alarmed by noises coming from upstairs. Believing there is an intruder, she dials 911, arms herself and slowly heads upstairs, discovering her husband, Ben, is having an affair with one of his students.
The Harmon family (Ben, Vivien, Violet, and their dog Hallie) head to Los Angeles, California, arriving at the same house as shown in 1978. While the realtor relays some of the house's history, Hallie starts barking uncontrollably at the basement door and Vivien instructs Violet to check on her. Curious, Violet goes down into the basement. The realtor discloses that the previous owners, a gay couple who restored the house to its original grandeur, died during a murder/suicide in the basement. Upon hearing this, Violet states, "We'll take it."
The Harmons unpack and move in, but it is apparent that Ben and Vivien are continuing to have marital problems, while Violet is regularly harassed by a trio of older classmates led by Leah. Vivien receives a surprise visit from her intrusive next door neighbor, Addy, and her mother, Constance. Constance gives Vivien a housewarming gift of sage. As Vivien burns it around the house, she makes her way to the attic, where she finds a bondage suit hanging from the ceiling. She asks Ben to get rid of it.
Ben begins sleepwalking while he's nude and hearing voices. He lights the fireplace downstairs, and is discovered by Vivien.
Vivien is hanging laundry when she is approached by Moira, whom she sees as an older woman in traditional French maid's uniform. Moira's intimate knowledge of the house (both how to care for it and its history) leads Vivien to hire Moira as a housekeeper. Moira describes that she was the housekeeper for the previous owners and is the one who discovered their bodies. Ben meets Moira in the kitchen, and he perceives her as a seductive, young woman in more tantalizing attire.
Ben expresses his concern about Tate, but is confident he can be treated. Unbeknownst to Ben, Tate has formed a strong bond with Violet. Ben is furious when he discovers Tate in Violet's bedroom and demands that he leave.
Coming out of a shower, Ben later walks in on Moira masturbating in a guest room. In his bedroom, he himself furiously masturbates. As he finishes, he starts to cry. He is startled by a man shouting, "Your family is in danger." He looks down to discover he is being watched by an extremely burnt man standing on the front lawn. Ben pursues the man, but finds he has fled.
Vivien is unloading groceries in the kitchen and hears a noise. She looks up, startled to find the cabinet and refrigerator doors have suddenly opened on their own. Addy giggles from another room, and sees the twins emerge behind Vivien (though Vivien is unaware). Vivien asks Addy to stop entering the house uninvited. Addy keeps asking if she can pet the dog. Vivien gently but firmly touches Addy's face to get her attention and to make sure that they are clear she should never come in the house uninvited again. Addy says okay and tries to touch the dog before leaving. It bites at her. Addy leaves angry, saying the dog shouldn't have done that. Constance threatens to harm Vivien if she ever lays hands on Addy again.
Ben tries to report Tate, believing that he poses a possible threat to himself and others, but is unsuccessful, as he does not have correct and complete information on Tate's background. Ben is distracted by Moira, who enters the room and tries to seduce him while he protests. They are discovered by Violet (who, like everyone but Ben, sees Moira as an older woman).
Violet is once again harassed by Leah at school. Tate and Violet plan to scare Leah. Violet lures Leah into the basement of her house with the promise of drugs. Tate is waiting there, and as Violet watches, Leah is attacked by Tate, who appears to Leah as the Infantata. Leah runs away, her face deeply cut. Violet argues that Tate took the prank too far and orders him to leave.
While restoring the music room, Ben attempts to seduce Vivien. They argue over her inability to forgive Ben for cheating. They reconcile through sex for the first time in almost a year. Later that night, as Vivien prepares for bed, she sees a figure in the doorway clad in the rubber fetish suit from the attic. She believes it to be Ben, unaware Ben is sleepwalking downstairs. Vivien and the suited man had sex, and Ben is saved by Constance (without memory of the incident) from burning himself on the stove while sleepwalking.
Ben is followed on a morning jog by the burnt man, who introduces himself as Larry Harvey, a previous owner of the house. Larry claims to be on release from prison due to terminal and inoperable brain cancer, and was apprehended after incinerating his family. He was left with burn scars on over 70 percent of his body as a result, compelled by the same whispering voices that Ben has been hearing. Ben threatens to commit Larry if he does not leave his family alone.
Moira catches Constance going through Vivien's jewelry box and, Constance responds by threatening Moira, "Don't make me kill you again."
Vivien comes home and tells Ben she is pregnant. Ben is overjoyed by the news.
- Adelaide: "Excuse me: you are going to die in there."
- Adelaide: "You're gonna regret it."
- Constance (to Moira): "Don't make me kill you again."
- Ben (to Vivien): "My son died too!"
Vivien (to Ben): "And you buried your sorrow in some 21 year old's pussy."
- Violet: "So we're the Addams Family now."
- Leah's friend Becca is played by Christian Serratos, who now plays Rosita Espinosa on "The Walking Dead".
- The end titles are sporadically inter-cut with images only a few frames at a time.
- Vivien looks over Ben's shoulder after telling him she is pregnant, and sees Rubber Man.
- Violet's face, in close up, horrified.
- Larry, staring up through the window.
- Addy, laughing at the Twins.
- Larry, driving after Ben.
- A mobile, made from dolls hanged from nooses.
- Vivien, having sex with Rubber Man.
- Moira's masturbation, and Ben's subsequent masturbation.
- Constance threatening Moira.
- Troy, covered in blood.
- Vivien, flanked by the Twins.
- The fire set by Larry.
- What is the connection between Tate and the entity in the basement?
- Who is the father of Vivien's baby?
- Why does Moira appear differently to Ben?
- What did Constance mean when she said she'd killed Moira?
- Who is Rubber Man?
- Is Tate actually alive?
- Main Title by Cesar Davila-Irizarry and Charlie Clouser
- Tonight You Belong To Me by Patience and Prudence (twin vandalism)
- Special Death by Mirah (comparing scars)
- Twisted Nerve by Bernard Herrmann (school shooting)
- Blood Gets Thin by Pete and the Pirates (meeting Leah)
- Baby, You Ain't Looking Right by Powersolo (closing credits)
- ↑ The time frame for the affair in relation to "the present" was established in Halloween: Part 2.
- ↑ The time frame for "the present" was established in Halloween: Part 1.