The film opens directly following the events of Evil Dead Rise. On a dark rural road, the newly possessed Jessica steps in front of a speeding car driven by Will, causing a catastrophic rollover. As Will is trapped and burning inside the vehicle, the decapitated Jessica chants ancient verses, passing the demonic corruption onto him before he dies.

Days later, Will's estranged wife, Alice, travels to a remote rural home for his wake and cremation. Still wrestling with complicated trauma from Will's abusive behavior, she reunites with his family: his brother Joseph, Joseph's girlfriend Thya, his parents Susan and Edgar, and his dementia-stricken grandmother Polly. At the private viewing in the family crematorium, Edgar visits the casket alone. Deadite Will bursts out, butchers the attendant, severely burns Edgar's arm, and slips into the night.

Believing Edgar suffered an accidental furnace burn, the family returns to the secluded estate, unaware that Edgar is already turning into a demon. As night falls and a heavy storm cuts off communication, Edgar succumbs entirely, brutally attacking the household. The curse spreads aggressively from person to person through blood, bites, and demonic manipulation. Joseph and Thya attempt to barricade the downstairs, but the Deadites toy with their minds, mimicking dead relatives and forcing gruesome choices.

Upstairs, Alice is attacked by grandmother Polly and seeks help from Susan, only to realize that Susan has spent years shielding Will's abusive behavior to protect the facade of a perfect family. Susan turns into a Deadite and attacks Alice, sparking a brutal brawl through the second floor. With the house engulfed in flames from overturned lamps and broken gas lines, Alice is forced to battle through her monstrous in-laws using hunting tools, blades, and fire.

In the climactic showdown inside the burning barn, Alice face-to-face destroys the remaining abominations, utilizing industrial farm blades and engulfing the structure in fire. Battered, bloodied, and burned, Alice crawls onto the nearby road as emergency sirens echo in the distance. When a paramedic asks who did this to her, she breathes out, "My ex-husband".

The ending highlights that the evil spreads like an airborne and blood-borne virus across the region once unleashed by the Necronomicon. Alice surviving the blaze represents her final, violent break from the cycle of domestic abuse and complicit family silence.

The mid-credits scene reveals that grandmother Polly survived the barn fire by violently severing her own pinned leg, leaping onto a passing motorist to continue spreading the plague into the next town. A secondary post-credits scene cuts back to the metropolitan crematorium in Los Angeles, showing the unclaimed ashes of Ellie Bixler reconstituting into a demonic force, teasing that multiple Deadite strains are running loose simultaneously.