Film student Tyler Tad Hilgenbrink is obsessed with The Hills Run Red, a horror film famed for its deranged masked killer Babyface and its vanished director Wilson Wyler Concannon William Sadler. Dubbed the scariest movie ever made, the project’s mystery pulls Tyler away from his girlfriend Serina Janet Montgomery, who ends up having an affair with their close friend, Lalo Alex Wyndham. When Tyler learns that Concannon’s daughter Alexa Sophie Monk works in a nightclub, he decides to seek her out to uncover the fate of the lost film. Alexa gives him a nude lap dance and later brings him home, where Tyler forces her to comply with him. She reveals that the film might be hidden at her father’s house in the woods.
Tyler, accompanied by Serina, Lalo, and Alexa, heads toward Concannon’s secluded residence, stopping to interview people along the way. He confides in Alexa that he believes Serina and Lalo are having an affair, and after a tense moment sharing his devotion to The Hills Run Red, he and Alexa share a kiss. The group is suddenly set upon by rednecks they had spoken with earlier, who threaten to harm Alexa and Serina. Babyface arrives and eliminates the attackers, then pursues the group into the woods before vanishing again. In a tense turn, Tyler fights his way into the house and discovers a red room filled with film reels hanging from the ceiling, before rescuing Alexa. The trio reunites with Serina, but Babyface ambushes them, rendering Tyler unconscious, capturing Alexa, and prompting Serina to flee—only to discover that Alexa has been manipulating the danger all along.
Inside the makeshift cinema, Concannon reveals the darkest truth: two decades earlier he stole Babyface’s costume from the original actor and murdered that performer to claim the mythic killer as his own creation. He has since killed every actor by making their on-screen deaths real, driving the film’s cancellations and the disappearances of the cast. He also discloses that Babyface is his son, born from an incestuous relationship with Alexa, who gave birth to him when she was twelve. Raised on The Hills Run Red, Babyface mutilated his appearance to resemble the character, a ritual that links his identity to his father’s cinema legacy. As Concannon speaks, Alexa begins filming a new movie while torturing Lalo in the barn, and Babyface sexually brutalizes Serina.
When Concannon summons Babyface, he returns to confront his father in the barn. Alexa, enraged by her father’s actions, shoots Lalo, and the two adults clash in a violent showdown. Tyler seizes a camera and uses it to persuade Babyface to kill Concannon, turning the killer’s loyalty against his creator. Babyface turns on Tyler, but Serina intercepts with a weapon and fatally stabs the monster. Alexa, furious at her son’s death, recovers and overpowers Serina and Tyler, tying Tyler up and forcing him to watch as she finalizes her cut of The Hills Run Red, thanking him for awakening her potential as a director separate from Concannon. The looping film continues as Tyler, now free of his gag, descends into maniacal laughter and bloodshed as he bleeds out.
In a mid-credits scene, Serina—still Alexa’s prisoner—is shown several months later, pregnant with Babyface’s child. Alexa carries Babyface’s mask as a grim gift for the future grandchild, singing a lullaby to the unborn baby while Serina screams.
