A con artist hawks musical instruments and band uniforms to small-town America.

"Professor" Harold Hill, a fast-talking traveling salesman, schemes to take money from the citizens of River City, Iowa, in the summer of 1912, by convincing them that a boys' band will put an end to the moral corruption resulting from the presence of a pool hall in the community. He sells instruments and uniforms to River City parents with the understanding that he will organize a band and teach the children to play; however, he intends to abscond with the money before the arrival of the instruments. Assisted by Marcellus Washburn, the town stableboy who helped Harold defraud customers in the East, Harold convinces the stubborn Iowa townspeople that he is an accomplished musician with a degree from a conservatory in Gary, Indiana. Marian Paroo, the piano teacher and librarian, uncovers evidence of his trumped-up credentials; but before she can tell Mayor Shinn, the owner of the pool hall, Harold charms her into believing he has developed the "think system" of learning to play musical instruments by humming Mozart's Minuet in G. Harold's influence transforms the quarreling school board into a harmonizing barbershop quartet, petty gossips into a ladies' dance committee, shy and lisping Winthrop Paroo into a happy child with dreams of playing in a band, and the high-strung "old maid" Marian into a young woman in love. The instruments and uniforms arrive, but Marcellus cannot convince Harold to leave town. On the evening of the ice-cream social, Charlie Cowell, a traveling anvil salesman, arrives in River City to expose Harold's scheme. Marian tries to stop him at first, but he tells her that Harold has romanced piano teachers in many towns. The alerted citizens chase Harold, planning to tar and feather him, but his love for Marian prevents him from escaping the mob. They handcuff the music man and bring him to Town Hall where the uniformed boys have assembled with their instruments. Harold pleads with the boys to employ the "think system"; and to his surprise, a ragged form of the Minuet in G escapes from the instruments. The shabby band becomes a splendid parade of color and music marching down the main street of River City, led by Harold with Marian at his side. Musical numbers: "Rock Island," "Iowa Stubborn," "Trouble," "Piano Lesson," "Goodnight My Someone," "Seventy-Six Trombones," "Sincere," "The Sadder-but-Wiser Girl," "Pickilittle," "Goodnight My Ladies," "Marian the Librarian," "Being in Love," "Wells Fargo Wagon," "It's You," "Shipoopi," "Lida Rose," "Will I Ever Tell You," "Gary, Indiana," "Till There Was You."