Teenager Maggie Thompson relocates with her mother Erica to her small-town Ohio home following the departure of Erica's husband. At her new school, Maggie quickly bonds with a group of friends, including Haley, Darrell, Chaz, and Andy—who develops an instant crush on her. They enlist the help of Sue Ann "Ma" Ellington, a local veterinary technician, to procure alcohol for their underage gathering. Sue Ann, however, chooses to anonymously report their antics, but they're let off the hook due to a friendly history between the police officer and Andy's father, Ben.
The very next day, Sue Ann invites the teens over to her house to party in her basement. What begins as a casual get-together transforms quickly as Sue Ann decorates her space and becomes a celebrated figure among high schoolers. Still, her overbearing hospitality soon becomes oppressive, leading the original group to feel increasingly uncomfortable with her presence. Tensions escalate when Sue Ann witnesses a romantic moment between Andy and Maggie, which ignites a mix of jealousy and rage within her—she harbors feelings for Andy, having once been infatuated with his father, Ben, during their high school years.
Motivated by jealousy, Sue Ann drugs Maggie, stealing her earrings in the process. When a confused Maggie awakens to find her earrings missing and bearing bruises, she confides in Andy, expressing her dread of returning to Sue Ann's house. To regain their trust, Sue Ann fabricates a story about having pancreatic cancer, using it as a cover for her erratic behavior. However, the teens quickly become suspicious upon noticing that Sue Ann sports a bracelet belonging to one of them.
Determined to uncover the truth, the girls decide to investigate and encounter Genie, Sue Ann's daughter from a failed marriage. To their astonishment, they discover that Genie, whom Maggie believed to be confined to a wheelchair, is in fact fully mobile. Just as they make their escape, Sue Ann returns home.
Tension mounts when Ben visits Sue Ann's workplace to drop off his cat, leading to a drink invitation. At the bar, he confronts Sue Ann with a tracking device, demanding to know why Andy frequents her house. Ben warns her to stay away from Andy, dismissing her as "pathetic." Flashbacks reveal a tragic past—Sue Ann had once performed a sexual act on Ben in high school, only to be betrayed during the encounter, leading to public humiliation that haunts her.
As Sue Ann spirals into madness, she commits further atrocities: running over Ben's girlfriend Mercedes and killing her boss. In a cruel twist, she extracts blood from Maggie's dog, Louie, planning to inject it into Ben. When Maggie confesses her fears about Sue Ann to Erica, she finds herself grounded. Shortly after, Sue Ann sends a distressing photo of herself and Andy at a party, prompting Maggie to confront him and discover that all their friends have been drugged.
Maggie’s horror escalates when she stumbles upon Ben's lifeless body. Chained in the basement, she witnesses Sue Ann torturing her friends—binding Haley's mouth and painting Darrell's face—a grotesque manifestation of her twisted mindset. Andy attempts to seduce Sue Ann to buy them time, but she stabs him, revealing her true nature.
When an officer arrives to investigate the commotion, Maggie’s screams for help turn tragic as Sue Ann shoots him dead. She orchestrates a macabre photo session, trying to recreate the friendship moments she never experienced, leaving Maggie in a perilous situation. Just as the house catches fire, Erica, knowing where her daughter would be, manages to save the group alongside her coworker Stu.
In a chaotic finale, Sue Ann attempts to sacrifice Genie, blaming Erica for the past's unresolved pain. In the heat of the moment, Maggie stabs Sue Ann, allowing Genie to escape safely. As the survivors grapple with the devastation left behind, they witness Sue Ann lying next to Ben's body in an embrace as their house crumbles into flames.
