In 1968, Bangor, Maine, teenage writing student Thaddeus “Thad” Beaumont suffers a series of painful headaches accompanied by the overpowering sound of chirping birds. During an operation, surgeons discover a tumorous growth containing an eyeball, nostril, and two teeth protruding from his brain. They deduce these are remnants of an underdeveloped twin absorbed by Thad during gestation. When one of the nurses excuses herself from the operating room, she and other staff members notice thousands of birds swarming outside the window. Twenty-three years later, Thad asks his wife, Liz, to read pages from his newest manuscript before leaving to teach his college creative writing class. In the lecture, he speaks of the duality of writers who must release their “inner being” in order for their work to flourish. Afterward, a man named Fred Clawson approaches Thad and threatens to reveal that the professor has published a series of lurid bestselling crime novels under the pseudonym “George Stark.” Fred demands money to remain quiet, and returns to New York City to allow Thad to think it over. Liz suggests he avoid controversy by revealing “Stark’s” identity to the press himself, but Thad worries that giving up his profitable alter ego could jeopardize his family’s financial stability. Convinced that Thad can successfully write under his own name, his publisher and agent agree to issue a press release exposing him as “George Stark.” Liz is relieved to see the end of the intense mood swings and alcoholic tendencies Thad experiences while writing as “Stark.” At the Beaumont’s cabin, Thad is interviewed by People magazine reporter Mike Donaldson, and explains the origin of “Stark’s” career. Mike laughingly suggests that Thad could be schizophrenic, and the photographer, Homer Gamache, stages a “gag” photograph of Thad and Liz posing next to “Stark’s” false tombstone with an epitaph that reads, “Not A Very Nice Guy.” That night, a physical embodiment of Stark rises from his grave and kills Homer, prompting an investigation by Sheriff Alan Pangborn. Although Thad was on a publicity tour in New York City at the time of the murder, Pangborn reveals that his blood and fingerprints were found inside the photographer’s truck. Believing he is being set up, Thad gives the deputies the name of blackmailer Fred Clawson, but police find him dead in his New York City apartment with the phrase, “The sparrows are flying again,” written in blood on his wall. As Thad works on his novel, he again hears birds and unconsciously scrawls the same words across his page. When Pangborn reveals that Thad’s fingerprints were also found in Fred Clawson’s apartment, Thad confesses to Liz that he has no recollection of writing the phrase, and worries that his tumor may be returning. Later, Thad is overcome by the chirping sounds, this time accompanied by Elvis Presley’s recording of, “Are You Lonesome Tonight.” Meanwhile, George Stark forces the former wife of Thad’s agent to leave a telephone message for Thad, then slits her throat. Thad reports the incident to Pangborn and provides a description of what he believes the killer looks like based on the identity he created for Stark. Meanwhile, Stark murders several police officers, Thad's agent, and Mike, the People magazine reporter. Stark then telephones Thad and instructs him to write another crime novel. Police trace the call to Thad’s own telephone calling card, prompting Pangborn to suspect Thad is orchestrating the murders with an accomplice. One day, Thad falls into a trance and telepathically communicates with Stark, who writes his responses by taking possession of Thad’s pencil. The script grows more erratic, until Thad eventually stabs himself in the hand. Before returning to his hometown, Thad asks Reggie Delesseps, a colleague at the university, to investigate the literary significance of sparrows. In Bangor, Maine, his childhood surgeon recalls the discovery of Thad’s partial twin and the swarm of birds that appeared during the operation. As Thad reviews his medical records, Stark discreetly murders the surgeon and escapes unnoticed. Thad telephones Liz to warn her of Stark’s close proximity, but Pangborn orders her to stay in the Beaumont family house under police protection. Reggie Delesseps tells Thad she believes he mentally willed Stark into existence, and that “the dark half” was embodied in the fetus of Thad’s former twin. She explains that sparrows are commonly viewed as “conductors of souls between the land of the living and the land of the dead.” Another swarm of birds circulates overhead as Thad learns that Stark has taken his family hostage. He arrives at their cabin to find Stark, the skin on his face partially decomposed, pointing a gun at one of his infant twins. Thad grabs the other child, and the two men head upstairs to the study, leaving Liz tied to a living room chair. As Stark furiously writes another book, his wounds transfer to Thad. However, Thad punctures Stark’s carotid artery with a pencil and he collapses on the floor, seemingly dead. Stark revives, but the birds’ chirping intensifies as thousands of sparrows peck through the walls and pick flesh from Stark’s body. Downstairs, Pangborn bursts through the front door and releases Liz, who runs to embrace her husband and children. Once Stark’s skeleton crumbles, the birds fly away, disappearing through a vortex-like portal in the night sky.