The film begins in 1912, showcasing a group of traveling salesmen on a train, who express their despair over societal changes that threaten their livelihoods. They disdain the notorious conman, Professor Harold Hill, notorious for selling musical instruments and uniforms to children while promising to form a band, only to vanish shortly after. As one salesman remarks, “He don't know one note from another!” Just before the train leaves River City, Iowa, Hill reveals himself, making a hasty escape.
Upon arriving in the quaint Midwestern town, filled with earnest and unpretentious residents, Hill is welcomed with open arms, much to his delight. He reunites with an old acquaintance, Marcellus Washburn, who now lives a respectable life in River City. Their conversation turns nostalgic as they reminisce about their past exploits in selling. Marcellus warns Hill that the town librarian, Marian Paroo, is too astute for his schemes, turning Hill's plans into both a professional and romantic challenge.
The ineffectual mayor's wife, Eulalie Mackechnie Shinn, makes her appearance soon after, criticizing Marian for recommending what she deems inappropriate literature for her daughter. Meanwhile, Hill cleverly identifies the installation of a pool table as his battleground, rallying the townsfolk with the catchy tune, “Ya got trouble / right here in River City.”
Marian, initially aloof, becomes subjected to Hill’s affections as he attempts to charm her with absurd pickup lines, which she swiftly rebuffs. Their encounters intensify as he tries to involve her in his dubious plans while simultaneously winning over the townspeople. Young Winthrop Paroo (played by Ron Howard), Marian's introverted brother who struggles with his lisp, finds newfound confidence with Hill's encouragement.
Love blossoms amidst the Fourth of July celebrations, where Hill's ambitions for a youth band are met with mixed reactions. As he fumbles through town, singing and dancing his way into the hearts of the locals, Marian's skepticism begins to wane, especially when she sees the positive changes in Winthrop inspired by Hill.
Tension mounts when the town's moral guardians, consisting of the school board and the mayor’s wife, demand to see Hill's credentials, leading to a series of chaotic musical numbers. His charm and quick thinking often save him from being exposed; however, as rumors circulate about his shady past, the stakes rise.
At a sociable gathering, amid an array of humorous antics and romantic tension, Hill’s affection for Marian becomes evident. Their duet, “Till There Was You,” highlights their growing relationship, but the dark undercurrent of his deception looms. When the town’s people uncover evidence of Hill's fraudulent activities, chaos ensues, leading to a confrontational assembly at the town hall.
As Hill's world begins to crumble, Marian courageously defends him, rallying the community to recognize the joy he has brought to River City. Over time, the assembled townsfolk come to acknowledge their experiences, leading to a heartwarming resolution. Ultimately, with the children's ragtag band on display and spirited performances, Hill finds redemption—not just through music, but also through the love and respect of the townsfolk and, more importantly, Marian.
In a triumphant ending, the once-chaotic band plays a harmonious rendition of “Seventy Six Trombones,” encapsulating the newfound spirit and unity of a town changed forever by Harold Hill’s charming deception.
