The story centers around "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. The musical explores how events unfold as cupid takes a part in the proceedings.

Main characters

• Prof. Harold Hill — a con man and traveling salesman who tries Iowa after saturating Illinois; falls for Marian
• Marian Paroo — the town librarian and part-time piano teacher; eventually falls for Harold Hill
• Marcellus Washburn — Harold's old friend, no longer a con man, who's dating his boss's niece, Ethel Toffelmier
• Mayor George Shinn — a proud local politician prone to saying nonsensical phrases, tries to control town but can't even control his own family
• Eulalie Mackecknie Shinn — the mayor's wife; a grand dame whom Harold appoints head of the "Ladies' Auxiliary Society for the Classic Dance"
• Mrs. Paroo — Marian's Irish mother
• Tommy Djilas — a young man "from the wrong side of town" who is secretly seeing Zaneeta Shinn; Harold encourages their relationship and makes him assistant band leader
• Zaneeta Shinn — the mayor's oldest daughter; secretly seeing Tommy Djilas
• Winthrop Paroo — Marian's shy, lisping, brother who hardly ever talks since the death of his father
• The Barbershop Quartet — four bickering businessmen and school board members united by Harold (Olin Britt, Oliver Hix, Ewart Dunlop and Jacey Squires)
• Pickalittle Ladies — Eulalie's four gossipy friends, Alma Hix, Mrs. Squires, Ethel Toffelmier and Maud Dunlop
• Amaryllis — Marian's young piano student
• Charlie Cowell — an anvil salesman who tries to expose Prof. Harold Hill as a con man
• Constable Locke — the town sheriff
• Gracie Shinn — the mayor's youngest daughter