On the eve of his twentieth anniversary as school principal, Seymour Skinner is lured by his mother to Springfield Elementary School for a surprise party. The celebration goes well until a strange man arrives, claiming to be the real Seymour Skinner. Principal Skinner soon admits that he is an impostor, and that his real name is Armin Tamzarian. Armin then tells the story of the events that led him to steal Seymour Skinner's identity.
Armin was once a troubled young man. He stole an old woman's purse and, while making his getaway on his motorcycle, hit a pedestrian who happened to be a judge. Armin's choices were either joining the Army and fight in the Vietnam War, go to jail, or apologize to the judge and the old woman. Skinner decided to join the army, not knowing the country was in the middle of a war. There he met and befriended the real Sergeant Seymour Skinner, who became his mentor and helped him find meaning in his troubled life. Seymour told Armin that his dream was to become an elementary school principal after the war. Later, Seymour was declared missing and presumed dead after he was caught in an explosion. Armin took the news of the apparent death to Seymour's mother, Agnes. Upon meeting him, however, Agnes mistook him for her son, and Armin could not bear to deliver the message. He instead allowed Agnes to call him Seymour, and took over Seymour's life. Meanwhile, the real Seymour Skinner spent five years in a POW camp, then worked in a Chinese sweatshop for two decades until it was shut down by the United Nations.
After these revelations, the people of Springfield begin to distrust Armin. Armin decides that there is no longer any place for him in Springfield, and retires from his job, breaks up with Edna Krabappel, and bids farewell to his surrogate mother. The real Skinner is then offered the chance to realize his dream and take over as school principal. He takes the job, but the real Skinner finds himself isolated by the townspeople after rudely berating Bart for insulting the Pledge of Allegiance, insulting his mother and acting sexist towards Edna. It turns out the people realize that they prefer Armin over him. Even Agnes Skinner misses Armin, since she had lived with him for 26 years, and does not believe her actual son still needs her. This was evident when she and the real Seymour fought over his curfew to be home for some family games. He defiantly responded by telling her to mind her own business and staying out at night going to bars. Armin, however, has already left Springfield and gone to Capital City to resume his old persona as a no-good street thug.
Marge heads to Capital City with Edna Krabappel, Agnes and the rest of the Simpson family, and Jasper. Armin refuses to return home, stating they have the real Skinner. However, Agnes refuses to acknowledge it, stating that Armin is her real son, having been her son longer than the real Skinner has, and neither she nor Skinner need each other anymore. After Agnes orders Armin to return home, Homer persuades Mayor Quimby and all the other citizens to allow Armin to resume his assumed identity as Principal Skinner. The real Skinner is unhappy about this, and refuses to give up his job and his dignity just because the people of Springfield prefer Armin to him. In response, the townspeople banish the real Skinner from town by tying him to a chair on a flatcar of a freight train, with Agnes telling Skinner she's happy he's alive, but he isn't the kind of son she needs. Judge Snyder declares that Armin will again be referred to as Seymour Skinner, that he will return to his job as school principal, and that no one shall mention the name "Armin Tamzarian" again, under penalty of torture.