In the early summer of 1912, aboard a train leaving Rock Island, Illinois, Charlie Cowell and other traveling salesmen engage in a heated argument about consumer credit ("Rock Island"). They eventually turn to another topic: a con man known as "Professor" Harold Hill, whose scam is to convince parents he can teach their musically disinclined children to play musical instruments. On the premise that he will form a band, he takes orders for instruments and uniforms. But once the instruments arrive and are paid for, he skips town without forming the band, moving on before he is exposed. Upon the train’s arrival in River City, Iowa, a stranger stands up and declares, "Gentlemen, you intrigue me. I think I shall have to give Iowa a try." Retrieving his suitcase, clearly labeled "Professor Harold Hill," he exits the train.
The townspeople of River City describe their reserved, "chip-on-the-shoulder attitude" ("Iowa Stubborn"). Harold stumbles across his old friend Marcellus Washburn, who has "gone legit" and now lives in town. Marcellus tells Harold that Marian Paroo, the librarian who gives piano lessons, is the only trained musician in town. He also informs Hill that a new pool table was just delivered to the town's local billiard parlor, so to launch his scheme, Harold convinces River City parents of the "trouble" that will be caused by that pool table ("Ya Got Trouble"). Harold follows Marian home, attempting to flirt with her, but she ignores him. At home, Marian gives a piano lesson to a little girl named Amaryllis while arguing with her widowed mother about her high "standards where men are concerned", telling Mrs. Paroo about the man who followed her home ("Piano Lesson/If You Don't Mind My Saying So"). Marian's self-conscious, lisping 10-year-old brother Winthrop arrives home. Amaryllis, who secretly likes Winthrop but teases him about the lisp, asks Marian whom she should say goodnight to on the evening star, since she doesn't have a sweetheart. Marian tells her to just say goodnight to her "someone" ("Goodnight, My Someone").
The next day is Independence Day, and Mayor Shinn is leading the morning festivities in the high school gym, with the help of his wife, Eulalie MacKecknie Shinn ("Columbia, Gem of the Ocean"). After Tommy Djilas, a boy from the wrong side of town, sets off a firecracker, interrupting the proceedings, Harold takes the stage and announces to the townspeople that he will prevent "sin and corruption" from the pool table by forming a boys' band ("Ya Got Trouble [Reprise]/Seventy-Six Trombones"). Mayor Shinn, who owns the billiard parlor, tells the bickering school board to get Harold's credentials, but Harold teaches them to sing as a Barbershop Quartet to distract them ("Ice Cream/Sincere"). Harold also sets up Zaneeta, the mayor's eldest daughter, with Tommy, and persuades Tommy to work as his assistant. After another rejection by Marian, Harold is determined to win her, telling Marcellus that she’s the girl for him ("The Sadder But Wiser Girl"). The town ladies are very excited about the band and the ladies' dance committee that Harold plans to form. He mentions Marian, and they intimate to him (falsely, as it turns out) that she had an inappropriate relationship with deceased old miser Madison, who gave the town the library, but left all the books to her. They also warn Harold that she advocates the "dirty books" by "Chaucer, Rabelais, and Balzac" ("Pick-a-Little, Talk-a-Little"). The school board arrives to collect Harold's credentials, but he leads them in song and slips away ("Goodnight, Ladies").
The next day, Harold walks into the library, but Marian ignores him yet again. He declares his unrequited love for her, leading the teenagers in the library in dance ("Marian the Librarian"). For a moment, Marian forgets her decorum and dances with Harold. He kisses her, and she tries to slap him. He ducks, and she hits Tommy instead. With Tommy's help, Harold signs up all the boys in town to be in his band, including Winthrop. Mrs. Paroo likes Harold and tries to find out why Marian is not interested. Marian describes her ideal man ("My White Knight"). She tries to give Mayor Shinn evidence against Harold that she found in the Indiana State Educational Journal, but they are interrupted by the arrival of the Wells Fargo wagon, which delivers the band instruments ("The Wells Fargo Wagon"). When Winthrop forgets to be shy and self-conscious because he is so happy about his new cornet, Marian begins to see Harold in a new light. She tears the incriminating page out of the Journal before giving the book to Mayor Shinn.
The ladies rehearse their classical dance in the school gym while the school board practices their quartet ("It's You") for the ice cream social. Marcellus and the town's teenagers interrupt the ladies' practice, taking over the gym as they dance ("Shipoopi"). Harold grabs Marian to dance with her, and all the teenagers join in. Regarding Winthrop's cornet, Marian later questions Harold about his claim that "you don't have to bother with the notes". He explains that this is what he calls "The Think System", and he arranges to call on Marian to discuss it. The town ladies ask Marian to join their dance committee, since she was "so dear dancing the Shipoopi" with Professor Hill ("Pick-a-Little, Talk-a-Little" [Reprise]). They have reversed their opinions about her books, and they eagerly tell her that "the Professor told us to read those books, and we simply adored them all!"
That night, the school board tries to collect Harold's credentials again, but he gets them to sing again and slips away ("Lida Rose"). Marian, meanwhile, is sitting on her front porch thinking of Harold ("Will I Ever Tell You?"). Winthrop returns home after spending time with Harold and tells Marian and Mrs. Paroo about Harold's hometown ("Gary, Indiana"). As Marian waits alone for Harold, traveling salesman Charlie Cowell enters with evidence against Harold, hoping to tell Mayor Shinn. He has to leave on the next train, but stops to flirt with Marian. She tries to delay him so he doesn't have time to deliver the evidence, eventually kissing him. As the train whistle blows, she pushes him away. Charlie angrily tells Marian that Harold has a girl in "every county in Illinois, and he's taken it from every one of them – and that's 102 counties!"
Harold arrives, and after he reminds her of the untrue rumors he's heard about her, she convinces herself that Charlie invented everything he told her. They agree to meet at the footbridge, where Marian tells him the difference he's made in her life ("Till There Was You"). Marcellus interrupts and tells Harold that the uniforms have arrived. He urges Harold to take the money and run, but Harold refuses to leave, insisting, "I've come up through the ranks... and I'm not resigning without my commission". He returns to Marian, who tells him that she's known since three days after he arrived that he is a fraud. (He said he was a graduate of Gary Conservatory, Gold-Medal Class of '05, but the town wasn't even built until '06!) Because she loves him, she gives him the incriminating page out of the Indiana State Educational Journal. She leaves, promising to see him later at the Sociable. With his schemes for the boys' band and Marian proceeding even better than planned, Harold confidently sings "Seventy-Six Trombones". As he overhears Marian singing "Goodnight My Someone", Harold suddenly realizes that he is in love with Marian; he and Marian sing a snatch of each other's songs.
Meanwhile, Charlie Cowell, who has missed his train, arrives at the ice cream social and denounces Harold Hill as a fraud. The townspeople begin an agitated search for Harold. Winthrop is heartbroken and tells Harold that he wishes Harold never came to River City. But Marian tells Winthrop that she believes everything Harold ever said, for it did come true in the way every kid in town talked and acted that summer. She and Winthrop urge Harold to get away. He chooses to stay and tells Marian that he never really fell in love until he met her ("Till There Was You" [Reprise]). The constable then handcuffs Harold and leads him away.
Mayor Shinn leads a meeting in the high school gym to decide what to do with Harold, asking, "Where's the band? Where's the band?" Marian defends Harold. Tommy enters as a drum major, followed by the kids in uniform with their instruments. Marian urges Harold to lead the River City Boys' Band in Beethoven's Minuet in G; despite a limited amount of traditional quality, the parents in the audience are nonetheless enraptured by the sight of their little boys playing music. Even Mayor Shinn is won over, and, as the townspeople cheer, Harold is released into Marian's arms ("Finale").
- From Wikipedia