THE play is set mainly in the little town of River City, Iowa, in 1912, but the opening scene is on a moving train in which the dialogue of travelling salesmen and the musical background beautifully simulate the bouncy rhythm of the jogging train. Aboard this train the salesmen are discussing the selling powers of one Harold Hill, without realizing that he is also one of the passengers. Harold Hill gets off at River City and comes to the centre of the town. There an old crony, Marcellus Washburn, tells him he will never be able to work his racket in this town: Harold Hill's racket is to go from town to town and influence its citizens to start a boys' band; then to abscond with the money the townspeople give him for the purchase of instruments and uniforms. Marcellus further informs Harold that the main obstacle in River City is the town librarian/music teacher, Marian Paroo, a "stuck up" sort of girl, who can be counted upon to see right through Harold's chicanery. Undaunted, Harold Hill proceeds to arouse the town's enthusiasm for starting a boys' band by pointing out the corrupt influence on their children of the local pool parlour ("Trouble"). When Marion Paroo appears, Harold tries to win her over with his charm, but she brushes him off rudely. But Marian is a soft and sentimental girl, as she reveals by speaking to the stars ("Goodnight, My Someone").

Inside the gymnasium of Madison High School a patriotic tableau is being given; Mayor Shinn follows with an address. Harold Hill demands the attention of the audience and starts once again to expound his ideas about a boys' band until the kids become infected with his enthusiasm ("Seventy-Six Trombones"). Later the same evening, Harold Hill goes to the library to win Marian over to his cause. He tries to impress her by telling her he is a "professor" of music, a graduate of the Gary, Indiana University, in the gold medal class of 1905. But Marian tells him in no uncertain terms that he cannot mesmerize or hoodwink her the way he had done the rest of the townspeople. Thus brushed off unceremoniously, Harold consoles himself with the idea that, after all, he has no possible interest in a girl as prim and conventional as Marian. ("The Sadder but Wiser Girl"). Nevertheless, he makes another effort to win over Marian, by sneaking up to her desk at the library and trying to convince her that he is infatuated with her ("Marian the Librarian").

But all the while, Harold keeps alive the enthusiasm for his band project, and goes about town signing applicants. Cuttingly, Marian asks him why he does not use his gift to greater advantage at a carnival. She knows he is a fraud, and thus will have no traffic with him, even though she is not the kind of girl who waits for a knight in shining armor ("My White Knight"). The reason Marian knows he is a fraud is because she has consulted the Indiana State Educational Journal and has discovered that Gary, Indiana University, had not even been founded in 1905. She is about to bring this information to the leading citizens of the town when Wells Fargo arrives with the musical instruments and uniforms, creating so much enthusiasm and excitement among her neighbors that she simply does not have the heart to disillusion them about Harold Hill. Indeed, now that Harold has, indeed, delivered the instruments and the uniforms without absconding with the money she is much more sympathetic to him; and there is no question in her mind that he is a man of considerable charm. Her resistance to him is finally broken: She comes to tell Harold she is in love with him ("Till There Was You"). Now Harold Hill gives himself up completely to making a success of the boys' band. In a stirring finale the boys appear in full regalia and sound their first raucous notes as members of the town band.