The story relates a crippled former astronaut's account of the terrifying change he undergoes after being exposed to an extraterrestrial mutagen during a space mission to Venus (similar to the canceled Manned Venus Flyby, but using a "Saturn 16" rocket). Arthur, the narrator, begins the story with his hands bandaged, and complains of terrible itching both after the mission and currently. The change takes the form of numerous tiny eyeballs that break out on his fingertips. These eyeballs act as the titular "doorway" for an alien species, allowing them to see into our world; only the eyeballs are imperfect windows, and distort everything that the aliens see into horrifying monstrosities which, Arthur perceives, they fear and hate intensely.
Soon, the alien presence is not only able to see through this doorway, but take control of Arthur's shattered body, using him to commit terrible murders. In a desperate attempt to maintain his humanity, Arthur douses his hands in kerosene and sets them on fire, only to find out that once the doorway has opened, it cannot be so easily closed. He manages to make the alien presence go away for nearly seven years. But after the eyes reappear on Arthur's chest, he reveals that he plans to kill himself with a shotgun to prevent the alien being from committing any more atrocities.