The tumultuous bond between Jane Coslaw, the film's narrator, and her paraplegic younger brother Marty takes a dark turn following a string of chilling murders in their quaint rural town of Tarker's Mills, Maine, beginning in the spring of 1976. The mayhem begins with the brutal decapitation of railroad worker Arnie Westrum at the hands of an unseen assailant. Simultaneously, pregnant Stella Randolph faces her end in a shocking murder in her own sanctuary, while the abusive father Milt Sturmfuller meets a grim fate in his greenhouse, closely followed by the tragic loss of Marty's closest friend, Brady Kincaid.
In the wake of these harrowing events, the townspeople band together to form a vigilante justice group, despite the efforts of local Sheriff Joe Haller to quell their rage. The sheriff's resolve begins to wane, especially after Brady's father Herb voices his disappointment over the lack of action. Even the well-meaning Reverend Lester Lowe cannot steer the townsfolk from their path of vengeance.
As the vigilantes scour the nearby woods for the perpetrator, they unwittingly walk into danger, resulting in further attacks and murders. Particularly affected is Andy Fairton, who, along with others, later denies witnessing anything out of the ordinary. Distraught, Reverend Lowe experiences a horrific dream where he presides over a mass funeral, only to see his congregation, including the deceased in their coffins, morph into werewolves and launch an attack on him. He wakes up in terror, pleading with God to "let it end."
With the rising tide of violence, the townsfolk face curfews that disrupt their Fourth of July festivities. The Coslaw family chooses to create their own celebration in their backyard, inviting Nan's brother, Red, who harbors a penchant for alcohol. Red gifts Marty a custom-built wheelchair/motorcycle, nicknamed the "Silver Bullet," along with fireworks, allowing him a joyous night. In a risky venture, Marty lights his fireworks under the cover of darkness, attracting the attention of the lurking werewolf, which confronts him. In a nail-biting escape, Marty manages to launch a rocket into the creature's eye.
Determined to uncover the truth, Marty seeks Jane's assistance in tracking down anyone who might have suffered a recent eye injury. Their investigation leads them to Reverend Lowe, who is missing his left eye. Aware that adults would be unlikely to believe his alarming claims, Marty resorts to sending anonymous notes to Lowe, warning him that he knows the truth and suggesting he should end his life to stop the killings. This prompts Lowe, in a fit of madness, to attempt running Marty off the road.
Trapped beneath a covered bridge, Marty faces Lowe, who, having lost his grip on sanity, attempts to justify his murderous acts as divine intervention. In a desperate moment, Marty calls for help from a passing stranger, providing a crucial lifeline. Together, the siblings work to convince Red of Lowe's dark connection to the murders and the threat he poses to Marty. With Red's influence, Sheriff Haller begins an investigation into Lowe.
That night, driven by anxiety and a desire for answers, Sheriff Haller discovers Lowe barricaded in his garage, desperate to control himself and prevent further bloodshed. However, in an unexpected twist, Lowe succumbs to his transformation and violently kills Haller with a baseball bat.
With the werewolf now fixated on them, Marty and Jane coordinate with Red to have Jane's silver cross and Marty's silver medallion melted down into a bullet meant to end their tormentor's reign. Under the light of the full moon, they wait in a heightened state of anticipation as their home is plunged into darkness when the werewolf attacks, injuring Red in the chaos. Marty displays remarkable bravery as he retrieves the silver bullet and fires it at the beast, hitting it squarely in the eye. As it collapses, the creature reverts to its true form—Reverend Lowe—as the siblings begin to process their harrowing ordeal.
In the aftermath, as they stabilize emotionally, Marty and Jane share a heartfelt embrace, whispering declarations of love, allowing Jane to express what she had once struggled to say.
