Freddie Trimball, Dodie and Betty Rogers and Lee Watson, students at San Juan Junior College, are staging a concert to raise funds for a memorial honoring the town's war heroes, when Roy Dunne and several other tough kids, who have stolen fireworks and are being chased by two policemen, run into the hall. To avoid detection, the gang starts to perform as a harmonica quartet. However, the boys are subsequently arrested and receive a thirty-day suspended sentence. In passing sentence, the judge comments on the town's lack of properly supervised recreational facilities. Freddie and his pals decide to use the memorial fund to start a youth center and approach the mayor, Phineas Wharton, Jr., about converting an old, city-owned warehouse. However, the crooked mayor wants the site for his own purposes and blackmails councilman Frank Peabody into selling the warehouse to him through an innocent third party, the mayor's nephew Alvin. When the students go to inspect the boarded-up warehouse, they find the harmonica quartet rehearsing there and Freddie sings along with them. Roy, the quartet's "manager," suggests that Freddie perform with his group. Later, when they discover that the warehouse is being surveyed for a remodeling estimate, Roy, Freddie and the others go to the mayor's home to find out what is going on. Wharton tricks them into believing that he is being blackmailed by councilman Peabody. After they leave, the mayor accuses Peabody of stalling in persuading his fellow councilmen to sell the warehouse. After Peabody refuses to work with Wharton any more, the mayor decides to hold a party to showcase an act Alvin has been pestering him to present and invites all the council members. In the meantime, the students disturb Peabody by leaving notes and making threatening phone calls. While the party is in progress, Peabody accuses the mayor of making the threats, and the students overhear him say that he will expose the warehouse deal to all present. However, Peabody is then knocked out by a door and, although Freddie and his friends revive him, they are too late to prevent the mayor from collecting his guests' signature to approve the deal. Wharton's father, a former mayor, is disgusted by his son's behavior and recruits Alvin to help him plot his son's downfall. At the warehouse, meanwhile, Freddie and Roy have a dispute and start to fight. The mayor arrives with Peabody and orders a passing policeman to arrest everybody for disturbing the peace and trying to wreck his building. Just then, Alvin shows up carrying envelopes for the mayor and Peabody from the mayor's father who bluffs Wharton into thinking that Peabody now has in his possession papers which implicate him in an earlier crooked deal. Peabody is instructed to open his envelope only if Wharton fails to turn over the paper containing all the signatures he collected. After the mayor announces that he will recommend to the council that the warehouse be released for use as a youth club, he tears up a paper purporting to contain the signatures, but which is actually another document. On the opening night of the youth center, Freddie and the others prepare to present a show, but Alvin, who is also scheduled to perform, is being held prisoner by the mayor who, having discovered his father has tricked him, threatens to close the center. Eventually, the mayor brings Alvin to the show, where he performs his impression of a daytime radio serial. Wharton then takes the stage and announces a change of heart and donates the warehouse for the youngsters' use. After their show climaxes with an appearance by Gene Krupa and his band, Freddie and his friends congratulate the mayor, while the mayor's father applauds from the audience.