Most of the townspeople are completely hoodwinked, but there's a possible danger to Harold's plan: Miss Marian Paroo, the local librarian and music teacher. She suspects that "Professor" Hill's blather is just that, and sets about to prove that he's up to no good. Harold decides that the only way to win her over and allay her suspicions is by romancing her. Shenanigans ensue.
Some of the famous songs from the musical are Ya Got Trouble, Seventy-six Trombones, The Sadder But Wiser Girl, and Till There Was You, to name a few. Here's Harold Hill near the beginning of the film convincing the denizens of River City that their sons are on the road to corruption:
The songs and scenes in this film never get old, and will leave you in awe of the talented performers who made the era of movie musicals so magical. They also have the unfortunate side effect of leaving me sourly contemplating the state of present day movies and the actors in them.
Here's a trailer for That's Entertainment: