After George Stark (Timothy Hutton) kills a bunch of people connected with his burial that are associates of Thad Beaumont (also Timothy Hutton), Thad travels to the hospital in Bangor where he had the tumor in his head removed to find out why he was hearing the bird sounds and experiencing the other strange phenomena. He believed the tumor was responsible for it.
The doctor there told him it was not really a tumor, but a twin brother that did not develop. His parents were rattled by this revelation, and they treated this unborn twin as human remains and buried him in their family plot. The doctor also tells Thad about how an enormous flock of sparrows attacked the hospital the day the unborn twin was removed. Later on, Thad meets up with his friend Reggie (Julie Harris), who tells him that Stark was able to come alive through using the unborn twin as a vessel and through Thad wanting him to be alive. Reggie tells him that George wanted his life, but since he wouldn't give it up, and the fact that if George actually killed him he would also be killing himself, he went about killing the people around him. Reggie also tells him that the sparrows are psychopomps," which are conductors of human souls back and forth from the lands of the living and the dead. Stark then calls, telling Thad he had his wife Liz (Amy Madigan) and their children, and that they'd be going back to the family's wooden cabin where he was conceived. As Thad is about to get in Reggie's car to drive to the cabin, he and Reggie see the sparrows, and Thad concludes that they've come to take one of them away. He asks Reggie who, and she tells him the loser, of course. Thad asks her why he is not aware of them, but Reggie doesn't know. Before Thad drives away, she tells him in the end, it's what you believe.
Thad arrives at the cabin to find Stark (who, because Thad doesn't want him around, is deteriorating in his appearance) and his wife and children there. Thad makes him promise that he won't hurt any of them if he writes a new book using Stark's name. Liz tells him that he doesn't want help writing a book, he wants to take over his life. Thad tells her that he knows what George wants, but what he and George do not know is if that this will really save him. He doubts it will, because he didn't want him around anymore. George tells him there's only one way to find out who is finished and who isn't, and so they head to the study to begin writing.
As they write, Thad takes an open opportunity to attack George, and the two start fighting. George pins him down and tries to shoot him, but Thad stabs a pencil in his neck multiple times, which causes him to fall down on the floor, seemingly dead. George eventually revives himself and attacks Thad, but thousands of sparrows begin breaking into the house, looking to claim George. After some more scuffling, the sparrows all burst in, and proceed to consume Stark down to the bones before carrying his remains to the land of the dead while Thad, Liz, and the recently arrived Sheriff Alan Pangborn (Michael Rooker) look on.
Thanks Tornado Dragon!