On a hot day summer classes are being held. The remedial math class contains thirteen girls and a nervous teacher named Ozawa. Tomoko looks outside to see the school's brass band. Nakamura wants to quit the band but is unable to follow through. The band leaves for a baseball game, and moments later, the lunch truck arrives. Tomoko, wanting to escape math, decides to deliver the lunches with her friends.
On the train ride, Tomoko eats a bento. They fall asleep and miss their stop. They lose some of the lunches in the fields while dodging a train. They meet Nakamura at the stadium and pass out the lunches. Nakamura demands to know where his lunch is, and Tomoko says she doesn't know. He then discovers rice on Tomoko's chin. He also notices that the band is sick as the lunches spoiled in the heat. All 42 and their teacher go to hospital. Tomoko watches it on the news and is petrified.
The next day Nakamura holds an audition for new recruits. He gets two punks and a shy girl named Sekiguchi, who plays the recorder. Nakamura is desperate, and he hears Tomoko and the other girls outside. He stomps out and berates them because they messed up the lunches. He tells them that they must fill in for the band. The girls try to refuse, but Nakamura threatens to rat them out if they don't join. To escape math the girls reluctantly join.
The girls start to clown around with the instruments, except for Sekiguchi. Nakamura has no control, until Sekiguchi accidentally knocks over some big band records. One rolls into the hands of the school's star baseball player, who hates Nakamura. While being confronted, Nakamura realizes that he can turn the girls into a big band as they are 8 people short of a brass band. He realizes that teaching brass band instruments to these girls will not be easy, as it needs to be done in time for the game next week.
As the week rolls on, Nakamura trains them physically to improve their lungs. Sekiguchi alone strolls through the tasks with flying colors. Tomoko faces some conflicts with Nakamura, but realizes that she must get along with him.
On the day before the game, the girls run through a jazz piece and are pretty good at it, although a little squeaky in some places. As they marvel at their work, all 42 brass band members walk in and take over again. Everyone except Sekiguchi is eager to get out, but once the girls step out of the building, they break down into tears because they realize that they liked playing in a big band.
When school starts up again, Tomoko passes by the band room as the band members run through scales. She asks the band teacher what happen to Nakamura, and she tells her he finally quit. The band teacher offers Tomoko a chance to play, and she begins to accept, but as she enters the room she sees Sekiguchi. Remembering how cruelly she treated her back when the band was still in business, Tomoko leaves. She sells her computer and her little sister's PS2 to buy a used sax (in poor condition). A while later all the girls meet to discuss how to raise some money to buy new instruments. Some ideas are brought up but they are unfeasible, and despite the fact that Nakamura's family is well to do, he can't pitch in because he's afraid he'll get a beating from his parents. Finally they settle on an idea: they decide to get jobs.
The girls work at a supermarket, and raise a lot of money, but Tomoko loses most of the money when she has to pay damages from accidentally starting a fire. As the girls leave, a shocking secret is discovered when it is learned that everyone except Tomoko, her best friend Yoshie, Sekiguchi, and Naomi have spent all of their hard-earned money on designer clothes. The girls who spent their money on clothes run off with the school's baseball players leaving the four with a small amount of money to continue the band on their own.
They decide to go pick matsutake mushrooms. In the mountains, though, there is a trespassing fine, and forest rangers are heading in their direction. They try to escape, but a boar attacks them. Naomi climbs up a tree and falls down onto the boar's head. The boar is killed on impact. The rangers find them, and they are rewarded for killing the crop-damaging boar.
They buy the instruments, but they turn out to be broken and old. However, the punks take them to the junkyard to have their ex-boyfriends fix them up. Their playing skills however remain poor.
They find that the math teacher Ozawa is a jazz fan who plays sax.
They play in front of the supermarket they got fired from. The others that had left see this, and are so moved that they rejoin the band.
Tomoko tells the others about a winter music festival. They convince Ozawa to conduct, and they go to the school roof to record an audition tape. They leave Tomoko in charge of the tape, but she forgets to send it. She turns it in late but gets a rejection. She decides not to tell anyone.
During a hair appointment, Nakamura discovers that Ozawa is not really a professional sax player and was privately taking lessons. Embarrassed, Ozawa confessed that he was never a good sax player and only learned to play to impress the school's music teacher. He makes Nakamura swear that he will never tell a soul. He also decides to back out of conducting.
On the train ride to the music festival, Tomoko sits alone in another car. Nakamura decides to go over and cheer her up. While the others think it's a confession of love, she says to him that she did not have the heart to tell anyone, so Nakamura tells them for her, which crushes their spirit. To make matters worse, the train is delayed by the snow. The band teacher finds them and rushes them by bus to the auditorium; they have a spot since another band could not make the competition.
They spill out onto the stage just as the announcer declares they would not attend, causing the audience much amusement. They set up and play a 15-minute concert. While wooing the crowd, Ozawa conducts and the punks' boyfriends try to get them to notice them. At the end, the performance turns out to be a success.