Act I
In the early summer of 1912, aboard a train leaving Rock Island, Illinois, Charlie Cowell and other traveling salesmen debate whether innovations are making their profession more difficult. "Professor" Harold Hill is raised as one whose sales skills make him immune from such changes ("Rock Island"). Charlie says that Hill is a con man who promises to form boys' marching bands, then skips town after taking payments for instruments and uniforms. Upon the train's arrival in River City, Iowa, a passenger leaves the train with a suitcase labeled "Professor Harold Hill".

After townspeople of River City describe their reserved, "chip-on-the-shoulder attitude" ("Iowa Stubborn"), Harold sees his old friend and shill, Marcellus Washburn, who has "gone legit" and now lives in the 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 can come from a pool table ("Ya Got Trouble"). Harold follows Marian home, attempting to flirt with her, but she ignores him. 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"; she mentions the man who followed her home ("Piano Lesson/If You Don't Mind My Saying So"). Marian's self-conscious 10-year-old brother Winthrop arrives home. Amaryllis, who secretly likes Winthrop but teases him about his lisp, asks Marian to whom she should say goodnight 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, Mayor Shinn and his overbearing wife Eulalie MacKecknie Shinn lead the festivities for Independence Day at the high school gym ("Columbia, the Gem of the Ocean") but are interrupted by a firecracker set off by troublemaker Tommy Djilas. 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 gets 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 ("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 imply (falsely, it turns out) that she had an affair with a now-deceased miser, who willed the library building to the town but left all the books to Marian. They warn Harold that she advocates "dirty books" by "Chaucer, Rabelais and Balzac" ("Pick-a-Little, Talk-a-Little"). The school board arrives to review Harold's credentials, but he leads them in song and slips away ("Goodnight, Ladies").

The next day, Harold walks into the library to woo Marian in earnest ("Marian the Librarian"). For a moment, she forgets her decorum and dances with Harold and the teenagers. Harold kisses her; when she tries to slap him, she accidentally 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 sets out 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.

Act II
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 only has a few minutes before his train leaves, but stops to flirt with Marian. She delays him so he won'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. (Harold earlier claimed to have graduated from the Gary Conservatory in 1905, but Gary, Indiana, was not founded until 1906.) 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 the boys' limited musical ability, the parents in the audience are nonetheless enraptured by the sight of their children playing music. Even Mayor Shinn is won over, and, as the townspeople cheer, Harold is released into Marian's arms ("Finale").