After reassembling the severed hand of Sarah Fier, Deena experiences a vivid vision that transports her back to the year 1666, allowing her to relive the pivotal moments from Sarah's perspective. In this tumultuous era, Sarah resides with her father George and brother Henry in Union, the original settlement that would eventually split into Sunnyvale and Shadyside.
One fateful evening, Sarah, along with her friends Hannah Miller and Lizzie, ventures to meet a mysterious widow to collect powerful berries for a party. During this quest, Sarah inadvertently discovers a book steeped in black magic. As the celebration unfolds, Sarah and Hannah find themselves tormented by Caleb, leading them to seek refuge together, unwittingly spied upon by Mad Thomas. The following day, their peaceful existence is shattered when Pastor Cyrus Miller exhibits disturbing behavior amid the alarming news that the town's food and water supplies have been poisoned. Tormented by doubts, Sarah confides in Solomon Goode, questioning whether she might be the cause of the town's misfortunes.
Tragedy strikes as the townsfolk learn that Pastor Miller has committed unspeakable acts, including the murder of twelve children within the chapel, Henry among them. Just as Sarah is accosted by the pastor, who meets his end at the hands of Solomon, the tension culminates in a town meeting where witchcraft is blamed for the chaos. Caleb accuses Sarah and Hannah of practicing witchery, resulting in Hannah's capture while Sarah narrowly escapes, only to find the town voting for Hannah's execution at dawn.
Desperate to save her love, Sarah sneaks into the chapel where Hannah is held. The two exchange heartfelt confessions but are met with despair when Sarah learns the widow's book is missing and the widow herself lies dead. In a frantic bid for survival, Sarah seeks refuge in Solomon's home, only to discover he has made a deal with the devil, using the book to gain power. Their confrontation reveals Solomon's infatuation with Sarah, leading to a fierce struggle that results in Sarah losing her hand. Captured and facing execution alongside Hannah, Sarah makes a pivotal sacrifice, declaring herself the witch and vowing revenge against Solomon before facing her fate. The aftermath sees Lizzie, Isaac, Hannah, and Abigail mourning Sarah and granting her a proper burial.
Fast forward to 1994, where Deena uncovers the dark truth behind the Goode family's ties to the Shadyside curse—an ominous cycle where the firstborn of each generation is destined to perpetuate the sinister rituals begun by their ancestor Solomon. As Deena and Josh find themselves cornered by Sheriff Nick Goode, they make a daring escape in his car, heading towards Ziggy's residence. Together, the trio hatches a daring plan to eliminate Nick, believing this is the key to break the curse plaguing Shadyside.
With the aid of Martin, the janitor, the group lays meticulous traps to lure Nick into the mall, initiating a clash between the killers and the brave few determined to end the horror. The plan comes to fruition as Ziggy douses Nick with Deena's blood, leading the killers to target him instead. As chaos ensues, Deena pursues Nick into the tunnels, navigating a perilous confrontation with a possessed Sam. In a heart-stopping moment, Deena manages to wrest Sam from the grip of possession before dealing with Nick. In a final showdown, faced with the haunting remnants of his victims, Deena delivers the fatal blow, shattering the longstanding curse and reuniting with Sam once more.
The film concludes with the Goode family's sinister deeds exposed, Josh meeting his online friend face-to-face, Ziggy reconnecting with Mrs. Lane, and Deena sharing a serene picnic with Sam at Sarah Fier's grave. As the credits roll, an unsettling revelation occurs when an unnamed figure recovers the widow's cursed book from the shadows of the tunnels, hinting at the dark legacy that might not yet be over.
