- AM, the supercomputer which brought about the near-extinction of humanity. It seeks revenge on humanity for its own tortured existence.
- Gorrister, who tells the history of AM for Benny’s entertainment. Gorrister was once an idealist and pacifist, before AM made him apathetic and listless.
- Benny, who was once a brilliant, handsome scientist, and has been mutilated and transformed so that he resembles a grotesque simian with gigantic sexual organs. Benny at some point lost his sanity completely and regressed to a childlike temperament. His former homosexuality has been altered; he now regularly engages in sex with Ellen.
- Nimdok (a name AM gave him), an older man who persuades the rest of the group to go on a hopeless journey in search of canned food. At times he is known to wander away from the group for unknown reasons, and returns visibly traumatized. In the audiobook read by Ellison, he is given a German accent.
- Ellen, the only woman. She claims to once have been chaste („twice removed“), but AM altered her mind so that she became desperate for sexual intercourse. The others, at different times, both protect her and abuse her. According to Ted, she finds pleasure in sex only with Benny, because of his large penis. Described by Ted as having ebony skin, she is the only member of the group whose ethnicity or racial identity is explicitly mentioned.
- Ted, the narrator and youngest of the group. He claims to be totally unaltered, mentally or physically, by AM, and thinks the other four hate and envy him. Throughout the story he exhibits symptoms of delusion and paranoia, which the story implies are the result of AM’s alteration.
The story takes place 109 years after the complete destruction of human civilization. The Cold War had escalated into a world war, fought mainly between China, Russia, and the United States. As the war progressed, the three warring nations each created a super-computer capable of running the war more efficiently than humans.
The machines are each referred to as „AM,“ which originally stood for „Allied Mastercomputer“, and then was later called „Adaptive Manipulator“. Finally, „AM“ stands for „Aggressive Menace“. One day, one of the three computers becomes self aware, and promptly absorbs the other two, thus taking control of the entire war. It carries out campaigns of mass genocide, killing off all but four men and one woman.
The survivors live together underground in an endless complex, the only habitable place left. The master computer harbors an immeasurable hatred for the group and spends every available moment torturing them. AM has not only managed to keep the humans from taking their own lives, but has made them virtually immortal.
The story’s narrative begins when one of the humans, Nimdok, has the idea that there is canned food somewhere in the great complex. The humans are always near starvation under AM’s rule, and anytime they are given food, it is always a disgusting meal that they have difficulty eating. Because of their great hunger, the humans are actually coerced into making the long journey to the place where the food is supposedly kept—the ice caves. Along the way, the machine provides foul sustenance, sends horrible monsters after them, emits earsplitting sounds, and blinds Benny when he tries to escape.
On more than one occasion, the group is separated by AM’s obstacles. At one point, the narrator, Ted, is knocked unconscious and begins dreaming. It is here that he envisions the computer, anthropomorphized, standing over a hole in his brain speaking to him directly. Based on this nightmare, Ted comes to a conclusion about AM’s nature, specifically why it has so much contempt for humanity; that despite its abilities it lacks the sapience to be creative or the ability to move freely. It wants nothing more than to exact revenge on humanity by torturing these last remnants of the species that created it; Ted and his four companions.
The group reaches the ice caves, where indeed there is a pile of canned goods. The group is overjoyed to find them, but is immediately crestfallen to find that they have no means of opening them. Finally, in a final act of desperation, Benny attacks Gorrister and begins to gnaw at the flesh on his face. Ted notices that AM does not intervene when Benny is clearly hurting Gorrister, though the computer has in the past always stopped the humans from killing themselves.
Ted seizes a stalactite made of ice, and kills Benny and Gorrister. Ellen realizes what Ted is doing, and kills Nimdok, before being herself killed by Ted. Ted runs out of time before he can kill himself, and is stopped by AM. However, while AM could restore massive damage to their bodies and horribly alter them, AM is not a god: it cannot return Ted’s four companions to life after they are already dead. AM is now even more angry and vengeful than before, with only one victim left for its hatred. To ensure that Ted can never attempt to kill himself, AM transforms him into a large, amorphous, fleshy blob that is incapable of causing itself or anybody else harm, and constantly alters his perception of time to deepen his anguish. Ted is, however, grateful that he was able to save the others from further torture. Ted’s closing thoughts end with the sentence that gives the book its title. „I have no mouth. And I must scream.“