The story begins with the narrator, a man of "a noble descent" who calls himself William Wilson, denouncing his profligate past, although he does not accept blame[dubious – discuss]for his actions, saying that "man was never thus [...] tempted before". After several paragraphs, the narration then segues into a description of Wilson's boyhood, which was spent in a "large, rambling Elizabethan" schoolhouse, "in a misty-looking village of England". The house was huge, with many jumbled paths and rooms, and situated on extensive grounds; the students were kept on site perpetually, however, hemmed in by a fence surmounted by broken glass, only being released for short, guided walks and church service.
William describes meeting another boy who shared the same name, who had roughly the same appearance, and who was even born on exactly the same date — January 19 (which was also Poe's birthday). The other William represents his only competition in academics, sports, and popularity. The boy seemed to compete with him so easily, however, that William thinks it "a proof of his true superiority; since not to be overcome, cost me a perpetual struggle". William's name (he asserts that his actual name is only similar to "William Wilson") embarrasses him because it sounds "plebeian" or common, and he is irked that he must hear the name twice as much on account of the other William.
The boy gradually begins copying William's mannerisms, dress and talk; although, by a "constitutional defect", he could only speak in a whisper, he imitates that whisper exactly. He begins giving William advice of an unspecified nature, which he refuses to heed, resenting the boy's "arrogance". One night he stole into the other William's bedroom and saw that the boy's face had suddenly become exactly like his own. Upon seeing this, William left the academy immediately, only to discover that his double left on the same day.
William eventually attends Eton and Oxford, gradually becoming more debauched and performing what he terms "mischief". For example, he steals exorbitant amounts of money from a poor nobleman by cheating him at cards and trying to seduce a married woman. At each stage, his double eventually appears, his face always covered, whispers a few words sufficient to alert others to William's behavior, and leaves with no others seeing his face. After the last of these incidents, at a ball in Rome, William drags his "unresisting" double—who was wearing identical clothes—into an antechamber, and stabs him fatally.