A class of schoolgirls are bored during their summer make-up class. When the school brass band leaves to perform at a baseball game without their bento lunches, Tomoko and the other girls persuade their math teacher, Mr. Ozawa, to let them deliver the lunches. On the train, the girls fall asleep and miss their stop. They walk back to deliver the lunches to the band, but they have spoiled in the summer heat, and all but their cymbal player, Nakamura, becomes sick. Nakamura holds an audition for band replacements to play at an upcoming baseball game. Only three girls audition: two former members of a punk band, and the shy Sekiguchi. Nakamura confronts the other girls, threatening to turn them in for the food poisoning in if they do not join. The girls have no musical experience and clown around with their instruments, except for Sekiguchi. As they are several members short of a brass band, Nakamura decides to turn the group into a big band and perform swing jazz. The girls train hard for the performance. Sekiguchi's talent inspires the others, and they come to enjoy playing. However, on the day before the game, just as the girls have become confident, the brass band members recover and the girls are devastated. As the new school year begins, Tomoko buys a saxophone and discovers Nakamura playing his keyboard. The members of the swing band gather at school and decide to buy their own instruments. The girls get supermarket jobs to earn money, but Tomoko and several others lose their wages when a cooking demonstration gets out of hand, triggering the store's fire sprinkler system. The remaining girls spend a day picking matsutake mushrooms, but are attacked by a boar; they kill it and claim reward money, as the boar had been destroying crops. With the money, the girls buy cheap damaged instruments, and the two rockers convince their ex-boyfriends, who operate a wrecking yard, to repair them. The group, now dubbed Swing Girls, play their first public show; the performance goes badly, but Sekiguchi is given advice by an anonymous jazz fan. When the group approach him, he runs away. They chase him to his home and discover that he is Mr. Ozawa, who possesses an extensive collection of jazz records. Assuming he is an expert saxophonist player, they convince him to lead the band. The band's skills improve and they record an audition tape for a music festival. They leave Tomoko in charge of the tape, but she sends it too late and the band is rejected. Tomoko is too embarrassed to tell the others. Nakamura discovers that Mr. Ozawa is not really a professional saxophonist, and he quits, embarrassed. On the train to the music festival, Tomoko confesses that the band have no place at the festival, and the train is halted by snow. However, their teacher Ms. Itami informs them that another band has cancelled due to the snow and rushes them to the festival by bus. The Swing Girls rush onstage just in time and perform their set, wowing the crowd.
