Castle Rock, Maine. 1968. 13-year-old Thad Beaumont is a junior high school student who wants to become a writer and spends many hours in his bedroom writing violent-themed stories. His mother buys him a typewriter to use and soon, Thad starts having violent headaches and hearing strange noises... like hundreds of small birds screeching. One day while waiting for the school bus, he collapses and is taken to the hospital where X-rays show "mass" in his brain that is thought to be a tumor which requires sugery. The exploratory operation reveals that the growth in his brain is actually a fetus complete with an eye, teeth, and fingernails. The doctor summaries that it is Thad's twin embryo which never developed and which Thad's foetus absorbed. The growth is removed, while at the same time, thousands of screeching birds fly around the hospital.
23 years later. A grown-up Thad Beaumont is a fairly successful writer who teaches an intermediate writing class at the local university. He is married to Liz and they have two twin children, a 10-month old boy and girl. One day after class, a strange man named Fred Clawson whom is a reporter from New York City approaches him and addresses Thad as "George Stark". In his office, Fred reveals that he knows that Thad has been using a pseudonym to write violent best-selling thriller novels, and demands blackmail money once a month to keep his mouth shut. After Thad confides in Liz about his predicament, he decides to "come clean" about Thad's dual idenity to the public to get rid of the blackmailer. Thad and Liz bring together a group of reporters and photographers where he publicy reveals his pseudonym as well as shows them the remote lake house where he writes his novels as George Stark. As a gag, Thad and Liz "bury" George Stark in the local cemetery.
A few days later, the local gravedigger Holt, calls the police when he discovers a large hole at the gravesite where the fake grave used to be; something has dug itself out. Soon, one by one, all the people who knew about Thad's alias are savagely murdered by an unseen assailant. First when the photographer, Homer Gamache, is attacked by a hitchhiker and beaten to death with his prostetic leg. When Sheriff Alan Pangborn calls upon Thad the next day, he suspects him to be the killer because of a witness who described the killer being simlar looking, and Thad's fingerprints are found at the crime scene.
Soon after, Thad's old headaches begin to return. When Thad learns that Fred Clawson has been found dead in his New York apartment and the words "the sparrows are flying again" are painted in the murder victim's blood at the scene of the crime, Thad becomes convinced that George Stark, his so-called "dark half", has somehow become embodied and is taking revenge against all those around Thad for being "killed off."
Next, 'George Stark' breaks into the New York apartment of Miriam, Thad's publicist, whom he forces her to call Thad and leave a message on his answering machine before he kills her. Back in Castle Rock, Sheriff Pangborn pays Thad another visit to inform him about the latest murder, but despite strong evidence which included Thad's fingerprints found at the scene, does not arrest him because of Thad's strong alibi.
Next, George Stark goes after Mike Donaldson, the alcoholic reporter by ambushing him in the hallway outside his apartment and beats him to death. After each killing, Stark phones Thad to infrom him about each killing. Stark then sneaks into Thad's publisher, Rick Cowley's apartment and slashes him to death as well, killing two building window washers who witness the killing. Stark calls Thad again and threatens to kill every he knows unless he begins writing novels as George Stark again. However, Stark begins to slowly decompose as his embodiemen is fading away. He realizes that he will decompose completely unless Thad begins writing under his name again.
Investigating George Stark's origins, Thad goes to see the former doctor who treated his "brain tumor" who confides in him about the undeveloped fetus. Stark arrives and kills the doctor and leaves Thad behind to discover the body... and make it appear that Thad is responsible for the murder. Thad flees from the police where Stark contacts him again and tells him that he has abducted his wife and twin daughter and son and threatens to kill them unless he meets him at the summer lake house and begins writing as George Stark again.
Thad confides in a old professor colleague, named Reggie, whom tells him that Stark's embodiment was the result of Thad encouraging his Stark to write and that in order for Thad to defeat Stark without killing himself, he must confront his "dark half". Reggie tells Thad that the appearance of the sparrows are the condutits of the bringers of life and death whom can either help Thad or kill him.
Thad drives out to the summer lake house where he finally confronts George Stark face-to-face (whom has now decomposed so badly that he is using bandages and dressing to keep his decomposing face intact). Taking him to his office, Stark forces Thad to start writing a new book so he can become powerful again. Thad tries to stall for time, instead discussing the violent nature of the thriller book. As Thad begins writing, he tells Stark to take over the writing as Thad takes advantage of Stark's weakened state to begin a fistfight with him. While the tied up Liz is outside the office, MILLIONS of sparrows decend upon the house to collect one of the two writers. Just when Stark seems to have the upper hand, Thad reveals that he allowed Stark to take over the writing so the sparrows will collect him instead. The small birds swarm into the house and literally consume Stark who screams in agony as the birds carry his remains away and dissapear into the moonlit night, while Thad and Liz and their twins are saved.