The narrator gives himself the pseudonym of William Wilson because he does not want to his name to sully the page. He does not discuss his later evil deeds, and chooses instead to tell the story of how he became wicked suddenly rather than slowly, in the way of most men. He is close to dying and longs for someone to sympathize with him and acknowledge the extent of his fall. From his family he has inherited an excitable disposition.
As a child, William is temperamental and selfish. His weak-willed parents are unable to control him, and he does as he wishes. William goes to school in a large, prison-like Elizabethan house in a soothing, quiet old village that is marked by an eroded Gothic steeple. The house is "old and irregular," and the boys are confined to its property behind iron gates with jagged spikes, except on Saturday afternoons, when they walk in the nearby fields, and on Sundays, when they go to church. The pastor, Dr. Bransby, is also the principal of their school, and the narrator notes the contrast between his dual roles as benign cleric and strict administrator. Meanwhile, the two-story school is built like a maze, and the schoolroom is old and somewhat dismal, with Gothic windows. The narrator remembers his childhood vividly, although very little of interest actually occurred.
In general, William claims superiority over his classmates, with one exception. This boy shares his name, arrives at the school on the same day, and has the same birthday, although the two are not related. He does not obey William like the rest of their classmates, but instead becomes an embarrassment to the narrator, who remains civil to his rival but fears him for keeping up so effortlessly. The other boy does not seek to beat the narrator and in fact more or less seems to like him, and he seems to allow the narrator to win their daily quarrels. This annoys William, but he cannot bring himself to hate his rival. In fact, William suspects, had the other boy not challenged his position, they would have been friends. As it is, the narrator respects and is almost inseparable from his counterpart while remaining resentful and uneasy.
William is fond of making jokes at the other boy's expense under the guise of good humor, but the recipient of the pranks is never upset by the jabs, except in one area: the rival cannot speak in a voice louder than a whisper, although the voice resembles the narrator's own. The rival's return pranks center on William's dislike of his boring name and his annoyance that someone else shares it. The shared name is reflected by a general similarity in appearance that could cause others to confuse them as relations, although no one ever comments on the likeness except for the narrator and his counterpart. The other boy often imitates the narrator to harass him, but no one except the narrator ever notices. The rival also tends to give the narrator advice, which the latter does not appreciate, although the advice is often good. After some time, William's ambiguous feelings towards his counterpart turn to hatred, at which point the latter begins to avoid him.
Once, after the two boys have a particularly severe argument in their fifth year at the school, the narrator dimly remembers his infancy and gains the feeling that he previously met the other boy, but ignores the presentiment. Soon afterward, William visits his rival's bed to play a prank on him, but when he sees the boy's face, he is startled to see that the boy's face is slightly different than usual and supposes that his constant imitation of the narrator must have altered his face when sleeping. William does not complete his prank and leaves the school, enrolling at Eton a few months later. By this time, he has mostly forgotten the incident and dismisses it while indulging in a profligate existence.
During William's three years at Eton, he has a small party which lasts until dawn. Drunken, he is about to declare a toast when a servant alerts him that someone wishes to speak with him. Curious, William goes to the entrance only to find a youth dressed in the same clothes who seizes his arm and whispers, "William Wilson!" in William's ear. The narrator is shocked into alertness, but the visitor leaves before he can recover. For weeks, William wonders about his counterpart's identity, and he discovers that the boy left the previous school on the same day as he did. William's confusion is interrupted by his move to Oxford where, lavishly funded by his parents, he falls into further vice, including gambling, thereby supplementing his income with the money of his weak-minded schoolmates.
After two years, William meets a rich young nobleman named Glendinning, who he judges to be stupid enough to trick into gambling away his money. He allows Glendinning to win some of his money before luring him to a higher stakes game at a party of eight or ten people. Glendinning drinks heavily and begins to bleed money until he doubles the stakes and loses. The nobleman loses his drunken flush and becomes extremely pale, and William learns that he has caused Glendinning's financial ruin. Everyone is regarding Glendinning with pity when a stranger suddenly enters the apartment and exposes William's cheating in a familiar whisper before leaving. The host offers William his cloak and asks him to depart; William realizes that his own cloak is already on his arm and that the second, identical cloak must have belonged to the stranger.
William flees England to the continent, but although he moves from city to city, the other William continues to follow him. William continues to ask himself the reason for the persecution and how it is that he never sees the other man's face. Harassed to the point of alcoholism, William begins to fight his terror, imagining that his follower diminishes whenever he grows firm, and he resolves to free himself. In Rome, he attends a masquerade held by the Duke Di Broglio, whose wife he finds attractive. He has been drinking and is seeking the duke's wife, who has previously informed him of her costume, when he hears the instantly recognizable whisper.
William's double is wearing a similar costume of a blue Spanish cloak and black silk mask. Furious, the narrator insults him and drags him into a side room and closes the door. He tells the man to draw his rapier and repeatedly stabs him until someone tries the door. He glances away and returns to find that the room has altered to produce what he at first assumes is mirror, which shows William his bleeding self. However, it is not a mirror image but rather his double, who has revealed his face that is exactly the same as William's own. Speaking in William's voice, the doppelganger tells him that he has won, but has murdered himself.
In "William Wilson," Poe explores the idea of a split identity, where the main character is shadowed from his early school years by his doppelganger, who shares his name and birth date and who challenges him from the outset by achieving as much as the narrator, but seemingly without effort. The split seems to have occurred during the narrator's infancy, but whether the division created a clean separation of Wilson's moral and immoral sides is suggested but not confirmed. On the one hand, the narrator admits that the other William displays affection toward the narrator and that he generally tries to give the original William sound advice. Certainly, the second man consistently appears at times when the narrator is about to fall into new areas of vice, such as alcohol, gambling, and finally adultery. In this sense, then, he seems to act in later years as the first William's conscience, the alter ego that William has pushed away into a new body.
Despite the aura of goodness that surrounds the second William and that is contrasted with the first William's descent into sin, certain aspects of the story suggest that the alter ego may not be an entirely well-intentioned figure. Although the narrator is hereditarily prone to moral weakness, the constant presence of his double does not have the effect of mitigating the narrator's temperament but instead gives him his first lesson in what it means to hate. At first, the narrator is unable to despise his rival because he subconsciously senses their connection, but the other William seems to go out of his way to goad the first William by reminding the latter of their similarities. In addition, the final murder of Wilson's other self is not a premeditated murder, but rather the effect of the frenzy caused by his appearance at the masquerade. The double serves not so much as William's conscientious angel as his unrelenting judge.
Interestingly, no one but William ever appears to notice or comment upon the similarity between the two characters, and no one observes the other William's frequent imitations of the original. Thus, their connection is private and based on a complex relationship of love and hate that echoes Poe's treatment of the subject in "The Black Cat" and "The Tell-Tale Heart." Here, as in "The Black Cat" and "The Tell-Tale Heart," the narrator's initial love turns gradually into hate and finally into murder, although of the three tales, "William Wilson" provides the most reasonable explanation of the shift in emotions. The narrator claims that he became evil very suddenly, but we can see a steady shift in his morality simply by observing his deteriorating relationship with his doppelganger.
The first-person narration of the story suggests that the narrator is the original William, who is hounded by his alter ego. According to this interpretation, the other Wilson is the magnet for the narrator's paranoia and the catalyst that draws out the narrator's innate evil. That being said, we must consider the possibility that the narrator is not the original Wilson at all and that it is the other Wilson who has lost half of himself. If we interpret the split personality in this manner, then instead of a shadowy semi-ethical figure who seems to know more than the narrator about the narrator's true self, we have a man who has lost the less disciplined part of himself and wishes to change his other half's immorality so that William Wilson can again be in a way complete. Nothing in the narration suggests that this understanding of the story is true, but it remains a possibility because of the unreliability of the narrator's point of view.
The carnival atmosphere and masquerade ball setting of the final act of the story recalls Poe's other stories "The Cask of Amontillado" and "The Masque of the Red Death." This story shares some characteristics with the former in that a murderer completes his deed against the ironic background of a cheerful and crowded carnival, but has more in common with the latter story, which features men that have run away in fear from a reckoning but who finally meet their destiny at a masked ball. The game of hiding and revealing of identities that a masquerade produces is appropriate to the story because the narrator and his other self cease to hide their identities from each other, and upon the reveal, the two bodies are symbolically reduced to one. The conclusion of the story simultaneously holds depressing implications for the resolution of the double identity problem. Instead of a two-part soul reconciling into one, the problem is only ended by the triumph of one half over the other.