Castle Rock, Maine, 1968. A 13-year-old Thad Beaumont is an aspiring writer in junior high, where he immerses himself in crafting stories filled with violence. His mother supports his passion by gifting him a typewriter, but soon, Thad begins to suffer from intense headaches and hears eerie noises reminiscent of small birds screeching. One fateful day, as he waits for the school bus, he collapses, leading to a hospital visit where X-rays uncover a "mass" in his brain, initially thought to be a tumor requiring surgery. However, the shocking truth unfolds during the operation—it’s actually a fetus, complete with an eye, teeth, and fingernails, which is identified as Thad's long-lost twin embryo that his body absorbed. The anomaly is successfully removed, only to be accompanied by a chilling spectacle: thousands of screeching birds swarm the hospital.
Fast forward 23 years, and Thad, now a successful writer, teaches an intermediate writing class at a local university while enjoying a life with his wife Liz and their two twin babies. However, his life takes a dark turn when a mysterious reporter, Fred Clawson, from New York City approaches him, referring to him as "George Stark." Fred reveals his knowledge of Thad's pseudonym used for violent thriller novels and demands monthly blackmail payments to keep his secret safe. After confiding in Liz, Thad chooses to publicly disclose his dual identity in a bid to shake off the blackmail. He arranges for reporters and photographers to witness him bury "George Stark" in a local cemetery as a humorous stunt.
But soon, the laughter turns to dread when the local gravedigger, Holt, discovers a gaping hole at the supposed grave, indicating that something has risen from the dead. As the deaths of those aware of Thad's alias begin to mount, each victim is killed in brutal and shocking ways, starting with the photographer Homer Gamache, who is attacked and beaten to death by a hitchhiker. Suspicions turn toward Thad, particularly when a witness claims to have seen someone resembling him at the crime scene, and his fingerprints are found there as well.
As Thad's old headaches resurface, he learns that Fred's lifeless body has been discovered in his apartment, with the cryptic message "the sparrows are flying again" scrawled in blood at the scene. Convinced that "George Stark," his dark alter ego, has come back to take vengeance, he finds himself trapped in a web of terror. Stark brutally murders Thad's publicist, Miriam, and threatens Thad with every subsequent killing, each more gruesome than the last, as Stark's presence looms ever larger.
Desperate to confront this nightmare, Thad seeks the help of a former doctor who treated his brain condition. Tragically, Stark arrives first, killing the doctor in a way that implicates Thad in the murder. Fleeing from law enforcement, Stark takes Thad's family hostage, demanding he return to writing as George Stark to satiate his murderous appetite.
With guidance from an old professor, Thad learns that Stark's embodiment is linked to his own writing, and in order to vanquish this dark half, he must face it. Journeying to the summer lake house, Thad comes face-to-face with Stark, who's now grotesquely decomposed and relies on bandages to maintain his appearance. When Stark commands Thad to write a new novel for him, Thad devises a clever stall. Just as Stark gains the upper hand, Thad reveals his ruse, allowing Stark to take control of the writing. The sparrows, symbols of life and death, descend upon the house, and as Stark screams in despair, they consume him, leaving Thad, Liz, and their twins safe under the moonlit night.
