The film opens with real estate agent Carl Gabriel Olds hosting an open house. After the last couple leaves, a man watches from the shadows and slips into the basement. Carl checks the basement but finds no one.
That night, Alice Rachel Blanchard returns home after a run and hosts dinner with friends including Jennie Anna Paquin. Alice confides that her ex-husband Josh Stephen Moyer thinks she is being overly emotional about selling the house. Alice falls asleep and wakes in the middle of the night. She notices Jennie’s car in the driveway. Alice searches the house and discovers Jennie dead in the garage, her throat slashed. As she screams, someone emerges from a crawlspace behind her and grabs her.
The next day, Lila Tricia Helfer arrives at the house and is greeted by David Brian Geraghty, who has chained Alice in the crawlspace. Lila approves of the house and proclaims that it is now home. David sets up a video camera in an upstairs window while Lila lounges in the hot tub. Josh arrives to retrieve his golf clubs and is surprised to find Lila in the tub. She explains that she is housesitting and proceeds to seduce Josh. David films the encounter, and when Lila is ready to climax, he hands her a kitchen knife. She uses it to stab Josh in the neck, leaving David to clean up the body.
During the day, Lila disappears, while David maintains the house. He lets Alice out of the crawlspace to eat and use the bathroom. He warns Alice that Lila would kill her if she knew Alice was still alive. David keeps Alice nearby while he works, always careful to stash her back in the crawlspace before Lila returns. One day, a pair of housecleaners arrive unexpectedly, and David kills them.
Lila and David host a dinner party with two couples. During the dinner, Lila reveals that David is writing a children’s story. As she describes the plot, it becomes clear that she and David are twins. After one couple leaves, Lila seduces the woman in the second couple. The man joins them in a ménage à trois as David sets up the video camera. He retrieves a kitchen knife and goes into the bedroom, where the couple starts screaming.
Alice grows more convinced that David is not an equal partner in his murderous relationship with Lila. She convinces him that they can run away together. Meanwhile, she secretly grabs a small kitchen knife and takes it with her to the crawlspace.
One day, Lila pretends to be a potential buyer and lets Carl take her on a tour. In the bedroom, she tries to seduce Carl, but he hears David hiding in the bathroom. She quickly slashes Carl’s throat and orders David to clean up the body before the blood soaks through the floor to the ceiling in the living room.
David tells Lila that all the bodies are starting to rot and that they will have to leave soon. He packs a suitcase with his belongings and some clothes for Alice. When Lila discovers the suitcase, she checks the coolers where David has stored the bodies, looking for Alice. Not finding her corpse, Lila peers through a hole in the crawlspace door. Alice stabs Lila with her knife. The two struggle, but Alice overpowers the severely wounded Lila.
One of Alice’s friends arrives to check on her, sees the blood and finds the chained Alice. He tries to cut through the chains with a hacksaw. He goes upstairs to call 911, but David kills him before the dispatcher can relay the police message.
David races downstairs to find Alice holding a knife to Lila’s neck. She threatens to kill Lila unless David releases her. Instead, David offers to help, hoping to leave with Alice. Lila, however, stabs Alice in the chest, and both she and David are forced to flee while Alice lies wounded on the floor as sirens approach. The film closes as Lila tours yet another new house before declaring to the realtor that it already feels like home.
