The story of Jimmy Walker who became mayor of New York in the '20s. Used by professional politicians and money-grabbers, Walker himself was "stupid but clean", although his open affair with Betty Compton cost him dear.

In the spring of 1925, New York State Senator James J. "Jimmy" Walker is summoned to the office of Gov. Al Smith, who makes a personal request that the popular Jimmy run for mayor of New York City. Smith and his associates in the Democratic Party are certain that Jimmy can easily win the election, thereby consolidating the Party's power in the state of New York. However, Jimmy, an Irish Catholic, is reluctant because he is estranged from his wife Allie, who tired of his womanizing and irresponsibility. Smith soon removes this obstacle to Jimmy's campaign when he reveals that Allie is waiting nearby, ready to lend her support and attempt a reconciliation. With his political mentor, Chris Nolan, and Allie at his side, Jimmy begins his campaign, taking his message to the streets of New York. Visiting the city's various ethnic neighborhoods, Jimmy delights the assembled crowds with his own composition, "Will You Love Me in December," which he sings in Yiddish, Polish and Italian. After winning the election by a landslide, Jimmy sets himself to the task of appointing his staff. The politically savvy Chris hands Jimmy a list of job candidates who are owed "favors," but Jimmy insists on hiring staff members on their own merits. At home, Allie rebuffs Jimmy's advances, informing him that his election to mayor has not restored his full marital privileges. That night, Jimmy visits a nightclub and later passes out drunk on a park bench, where he is discovered by young nightclub singer Betty Compton, a Canadian emigré with a fondness for stray cats. Unaware that Jimmy is the mayor, Betty attempts to help him home. Shortly after, upon learning of Jimmy's identity, Betty angrily denounces his behavior as unbecoming to public office, and Jimmy finds himself smitten by her spunk. Using his power as mayor, Jimmy forces Broadway producer Bernie Williams to hire Betty for his new show. At first angry at Jimmy for his machinations on her behalf, Betty is eventually won over by his charm and sincerity. Back at City Hall, Jimmy proclaims that he will wipe out corruption in the city government. Chris, angry that Jimmy has not consulted Tammany Hall about his plans, condemns him as hopelessly naïve. As his first term comes to a close, Jimmy finally realizes that he will have to play the political game if he is to stay in office and fulfill his promises to the electorate. In the fall of 1929, Jimmy and Chris prepare for Jimmy's reelection campaign against Congressman Fiorello LaGuardia, who has released newsreels dismissing Jimmy as the "musical comedy mayor" and accusing him of spending one-third of his term on vacation while giving himself a hefty salary hike. Although LaGuardia's accusations are not without foundation, Jimmy's charisma overshadows his weaknesses as mayor and he is reelected in another landslide victory. Now broke as a result of the stock market crash, Jimmy retains his happy-go-lucky attitude, but Betty, tired of being hidden from public view, is bitterly disappointed by his reelection and the prospect of four more years of secrecy. Jimmy, who cannot persuade Allie to divorce him, then takes Betty to his victory party over the protests of his handlers. Soon after, the committee of Judge Samuel Seabury begins investigating allegations of graft and corruption in Jimmy's government. Chris and Charley Hand, Jimmy's loyal secretary, beg Jimmy to give up Betty so that he won't lose the Church's support, but Jimmy refuses, leading Charley to resign. With much dignity, the long-suffering Allie warns Betty not to cast her lot with Jimmy, who will never be happy out of the public eye. That evening, at a boisterous roast in Jimmy's honor, Betty realizes that Jimmy will never give her the stable home life of which she dreams and tearfully breaks off their relationship. After Betty attempts suicide, Chris hustles her onto a boat headed for Havana, and she later marries a young suitor. Jimmy, heartbroken over the loss of Betty, is summoned by the Seabury Committee and Gov. Franklin D. Roosevelt to testify about stocks and bonds he accepted from various political associates, and, in August 1932, he travels to Albany with Allie and Chris. Speaking in his own defense, Jimmy admits that he accepted monetary gifts, but claims that he never intended any wrongdoing and was guilty only of stupidity. After reciting the tale of "Susanna and the Elders," Jimmy goes on to proclaim that if he is guilty, then so are all of the politicians present. On the train back to New York City, Allie and Chris praise Jimmy for his honesty, but Charley, who now works for Roosevelt, denounces Jimmy for causing a split in the Party vote, which may damage Roosevelt's chances in the Presidential election. Jimmy, sure that he still has the support of his constituents, attends a Yankees baseball game, but is loudly booed by the spectators. From the field, Jimmy makes a resignation speech in which he apologizes for any wrongdoing he may have committed, but also reminds the spectators that they bear some of the responsibility by electing him in the first place. After saying goodbye to a tearful Allie, who still loves him and refuses to divorce, Jimmy boards a boat bound for Europe. As he gazes at the skyline of his beloved New York, Betty, who has gotten a quick divorce, arrives and declares her intention to spend her life with him, no matter where the boat should take them.