Stephen Byerley is a man who was the victim of a car accident, in which his wife died and he was severely wounded. After a slow recovery, he has returned to public life. Currently, he is a lawyer, a successful, middle-aged prosecutor, a humanitarian who never presses for the death penalty. He runs for mayor of a major American city (implied to be Chicago), but Francis Quinn's political machine smears him. They claim that the real Stephen Byerley, rather than having recovered, was permanently disfigured and crippled by the accident, and the Byerley who appears in the public is a humanoid robot, that is, a machine built to look like a human being, created by Stephen Byerley to be what he himself could be no longer. If this is true, the "Frankenstein complex" hysteria will ruin his campaign, as only human beings are allowed to run for office. Quinn approaches U.S. Robots and Mechanical Men corporation, the world's only supplier of positronic robot brains, and attempts to persuade them that Byerley must be a robot. No one has ever seen Byerley eat or sleep, Quinn reports.
All attempts to prove or disprove Byerley's humanity fail. He is visited by Alfred Lanning and Dr. Susan Calvin on Quinn's suggestion, and the Chief Robopsychologist offers him an apple. Quite nonchalantly, Byerley takes a bite — proving nothing since (like R. Daneel Olivaw) he may have been designed with an emergency stomach. Quinn attempts to take clandestine X-ray photographs, but Byerley wears a device which fogs the camera. Through all these investigations, Byerley remains calm and smiling, pointing out that he is only upholding his civil rights, just as he would do for others if he is elected. His opponents claim that, as a robot, he has no civil rights, but Byerley counters that they must first prove that he is a robot, before they can deny his rights as a human — including his right not to submit to physical examination.
Once all physical means are exhausted, Susan Calvin indicates that they must turn to the psychological side. If Byerley is a robot, he must obey the Three Laws of Robotics. Were Byerley to violate one of the Laws, he would clearly be a human, since no robot can contradict its basic programming. However, if Byerley obeys the Laws, it still doesn't prove he is a robot, since the Laws were invented with human morality in mind. "He may simply be a very good man," observes Dr. Calvin.
To prove himself to be a human being, Byerley must demonstrate that he is capable of harming a human.
Byerley never confirms or denies his flesh-and-blood status and lets the entire campaign ride on this single issue. While he is giving a speech, a heckler rushes the stage, and the heckler asks to be hit in the face. Byerley complies and punches the heckler in the face. Most people are convinced that he is human, and the emotional uproar demolishes Quinn's smear campaign. Byerley wins the election without further difficulty.
In the final scene, Susan Calvin confronts Byerley, who is again spending a late night awake. She says that she is somewhat regretful Byerley turned out human, because after all, a robot would make an ideal ruler, one incapable of cruelty or injustice. In an almost teasing speech, quite unlike her usual self, Dr. Calvin notes that there is one case, "just one", where a robot may avoid the First Law: when the "man" who is harmed is merely another humanoid robot. This implies that the heckler whom Byerley punched may have been a robot, and if that was the case, Byerley hadn't broken the First Law, leaving the question of his humanity open. In the binding text of I, Robot, which is not part of the actual story, Dr. Calvin notes that Byerley had his body atomized upon his "death" thus wiping out any evidence either way.
Several earlier scenes interspersed through the story, in which Byerley meets with his old "teacher", now take on new significance.
As she leaves Byerley, Dr. Calvin promises to vote for him when he runs for national office. Asimov's later story "The Evitable Conflict" reveals that he prospers in politics, eventually becoming head of the planetary government.