Act One

Mayor Fiorello H. LaGuardia reads the comics over the radio to the children of New York City during the newspaper strike. As he reminisces about the city's corrupt past, we travel back in time to 1915 to Fiorello's Greenwich Village law office. The usual congregation of impatient citizens waits to speak to Fiorello. Neil, Fiorello's law clerk, and Morris, his office manager, assure their visitors that Fiorello will help them ("On the Side of the Angles"); Fiorello will aid any person in need who comes to him, no matter his finances or stature.

Fiorello's secretary, Marie, comes into the office to greet her friend, Dora, who brings news of the latest crisis: Thea, President Wilson's daughter, has been arrested for helping the women shirtwaist strikers. Fiorello rushes in, ignoring everyone's problems for the moment. He needs Marie to introduce him to Ben Marino, the leader of the Fourteenth District who is looking for a candidate for Congress. Then, true to his nature, Fiorello promises to help the women strikers and the citizens waiting in his office.

Later that that day, Ben Marino and his cronies play their usual poker game ("Politics and Poker"). Marie arrives with Fiorello and convinces Ben to back Fiorello in beating the corrupt political machine of Tammany Hal,l which dominates city politics.

Meanwhile, Dora and her fellow workers walk the picket line in front of their factory ("Unfair") as they suffer the jibes of hecklers. Floyd, the policeman on duty, gets into a fight with Dora. Just as Floyd is on the verge of arresting Dora, Marie and Fiorello arrive on the scene. To the deep admiration of Marie, Fiorello protects the women's rights and chases away the hecklers. His duty done, he timidly asks Marie to dinner. On the heels of her accepting his invitation, Thea arrives. Just out of jail, she is anxious to know how Fiorello can help them win the strike. She asks him to take her out for dinner. For the sake of the cause, he must accept.

Back at the office, Marie receives a call from Fiorello, telling her that he must cancel their dinner plans. She reveals to Morris her disappointment and feelings for Fiorello. She feels like such a fool for caring so much while Fiorello cares so little ("Marie's Law").

A year later, Fiorello is actively campaigning for Congress, with Ben as his backer. Neil warms up the crowd that has come to hear Fiorello. Dora re-meets Floyd, the policeman who almost arrested her. Animosity evaporates as Floyd congratulates her on winning the strike. Thea then introduces the crowd to Fiorello. Fiorello takes the platform by storm, promising to break the stranglehold of Tammany Hall ("The Name's Laguardia!"). And, guess what? Fiorello wins the election ("The Bum Won")!

One year later, Dora and Floyd are an item ("I Love a Cop"). On her way to Washington, D.C., to see Fiorello, Marie stops by at Dora's. Marie reveals to Dora that Ben's convinced her to try talking Fiorello into halting his Draft Act rhetoric. It's obvious to Dora that Marie still has feelings for Fiorello, but she refuses to express them. Once in Washington, neither Marie nor any of Fiorello's associates can dissuade his avocation for America joining the war. In his true fanatical fashion, Fiorello plans to make a grand statement to Congress. He's also enlisted in the Air Force.

Back in New York, Ben gives Fiorello a big send-off party. As expected, each person has his or her own feelings about the war. Floyd is upset that the army won't accept him, as his feet are too flat. Marie is upset that she might never see Fiorello again. Fiorello, confident that he will return, asks Thea to marry him. She needs time to think about it. Marie and Fiorello share their own goodbyes along with the rest of the somber partygoers ("'Till Tomorrow").

When the war is over, Fiorello, along with thousands of other servicemen, arrives home ("Home Again"). Both Marie and Thea wait for him at his ship's gangplank. He rushes straight to Thea, who has accepted his marriage proposal. Morris comforts Marie as she turns away from the happy couple.

Act Two

It's 1929, and Fiorello is running to be the Mayor of New York City. He and Thea have made themselves a happy home. Since Floyd and Dora have gotten married, Floyd has long since left the police force, and his alternate careers flourish. Thea has just made a visit to the doctor – the outcome of which she keeps from Dora and Fiorello. Even she can't believe how much she loves Fiorello ("When Did I Fall in Love").

At Dora and Floyd's penthouse apartment, there is a meeting of the corrupt backers of Fiorello's competitor for mayorship, Jimmy Walker. Floyd's boys have invited a group of chorus girls who are to perform for the campaign ("Gentleman Jimmy"). A mobster has the idea to have Fiorello killed as he gives his speech to the public. This is information that Dora can't keep to herself. She rushes over to Fiorello's headquarters to talk to Marie in confidence. Fiorello's office is having its own set of problems. Ben has been outspoken over the way Fiorello's handling his campaign, and Fiorello fired him. Fiorello's concerned over his wife's health, which appears to be deteriorating.

On the day of Fiorello's public speech, his opposition's plan to kill him is foiled, but Thea dies. Maria waits until the end of the day to tell him. Jimmy Walker wins the race for mayor. Instead of being crushed by personal tragedy, Fiorello is inspired to renewing his energy in his work.

It's now 1933; Ben and his political cronies play poker in his old office ("Little Tin Box"). Marie makes a visit. She has decided to quit her job and marry the very next man who asks her ("The Very Next Man"). But, before she leaves Fiorello's office, she must persuade Ben to return to Fiorello and help him win the upcoming mayoral election. She then returns to Fiorello's headquarters and convinces Fiorello to overcome his fear of losing a second time to run for mayor once again. Not only does he concede to run, with Ben's backing, but he also asks Marie to marry him. She accepts, of course, and, as history tells, Fiorello finally wins the race for mayor.