The story centers around five Japanese high school students who are too young to be called adults, but who no longer think of themselves as children. Sakai Wakana once took music lessons, but she withdrew from music after losing her mother. Miyamoto Konatsu is a positive-thinking girl who loves singing and spends time after school at the vocal music club. Okita Sawa is a spirited archery club member who dreams of becoming a horse rider. Tanaka Taichi is a chronically late badminton team member who lives with his college student sister. Wien just transfered into Wakana's class after 12 years abroad in Austria. Music brings Wakana, Konatsu, Sawa and the others together into an ensemble during their last summer in high school.
As the gang and their families prepare for the festival, they discover the school gates are closed and the gym has been locked up. As the chairman confronts them, Tayoru decides to stand up to him and allow the festival to be held, with Naoko also offering her support by enlisting the help of the vocal group and wind instrument club to help perform Wakana's song. The festival soon gets on the way and the gang perform their musical in front of many guests. After the festival, Wien finally receives a postcard from Jan, whilst Sawa goes abroad to attend an equestrian school, much to the dismay of Taichi who had developed feelings for her. Later, Wakana and Naoko get together to reminisce about Mahiru, with Wakana asking Naoko to teach her more about music. Soon after, everyone graduates and start moving towards their futures.
As Konatsu reveals the club will perform a musical drama at the school festival, Wien seems preoccupied by something. Meanwhile, Shiho suggests putting on a superhero show at the shopping district and gets Konatsu and the others to play the part of the superheroes, sparking Wien's interest in particular. As Wakana asks Shiho about how Mahiru went about writing her songs, she suggests asking Naoko, as she previously co-wrote a song with her. As the others notice Wien taking the superhero show a bit too seriously, he reveals all the letters he sent to a young kid in Austria named Jan were sent back to him. Meanwhile, Naoko is worried about some supposed changes planned by Tayoru's superiors.
Despite some initial setbacks, the gang manage to make the superhero show a success. The next day, Naoko calls up Konatsu and co about their job, but they manage to win the argument as she had already given them permission. Afterwards, Wakana asks Naoko about how Mahiru wrote her songs, to which she replies that she needs to enjoy it whilst also thinking of her own memories with Mahiru. As the gang continue doing the shows, Konatsu's bag is stolen by a thief on a bicycle, prompting Wien to chase after him. With the support of his friends, Wien manages to catch the thief and together they make the headlines.
As Konatsu meets the Condor Queens, which was the band that got her into music, Wien spots Wakana being chased after by the foreigner, who turns out to be the third member of Condor Queens and an acquaintance of Wakana's late mother, Mahiru. Meanwhile, Sawa starts to become annoyed with Konatsu as she appears to want to spend more time with the Condor Queens than practise for the choir club, threatening to quit if she doesn't take things seriously. Wanting to make amends, Konatsu searches for a stage to perform on, which is thankfully registered by Wakana, and manages to make up with Sawa. On the day of the music festival whilst the others perform on their modest stage, the Condor Queens give Wakana a letter Mahiru gave them when she was born.
As the choir club works hard on its musical, Taichi, who is tasked with creating background props, tries to recruit a decent painter, finding one who'll agree on the condition he takes some photos of Sawa. As Konatsu tries to get help from the vocal group, she hears from Naoko that the school festival has been cancelled as the school is being closed down to make way for new buildings. Just as the group looks to be split up, Wakana reveals she has finished writing her song, deciding they should have their own festival so they can sing it together.
As Wakana comes down with a cold, Konatsu decides to pay her a visit, reminding Wakana of her promise with Mahiru to write a song together. Later, Wakana receives a tape from Shiho containing a recording of when Mahiru was in the choir club before Sawa takes her horse riding. The next day, Wakana helps Taichi and Atsuhiro with their vocal training before visiting Tomoko and her new baby. Later that day, Wakana learns from her father why Mahiru kept her illness a secret from her; because she didn't want the song they wrote together to be a sad one. As Wakana laments how she allegedly threw away her mother's feelings, her father gives Wakana some sheet music Mahiru wrote before she died so that she can finish it herself, admitting he never sold the piano. As Wakana listens to the tape the next day, she rekindles her own love of singing and joins the rest of the club in song.
As the club tries to decide what to do for the school culture festival, Konatsu is taunted by her former clubmembers. Konatsu soon learns that, in order to get on the main stage for the festival, they will have to be approved by Naoko. Meanwhile, Sawa is at odds with her father, who sees her ambition of becoming a professional jockey as simply playing around. For some unknown reason, Sawa starts going a diet and begins skipping a lot of meals, causing Konatsu and Wakana to worry about her. After the girls attend a musical together, Wakana looks to learn composition in order to finish the song Mahiru was writing whilst also considering her own career. Meanwhile, Sawa is left fatigued from her forced dieting and ends up falling off her horse during mounted archery practise.
After being checked by the hospital, Sawa is told by her father to quit horseback archery. Sawa later reveals to Shiho that she was skipping meals as she was attempting to apply for a riding school, but wasn't accepted due to her weight and height. The ordeal leaves Sawa a bit shaken and emotionally fragile. With the vocal group hoarding the practise rooms, the choir club goes to Wein's house, where Sawa explains her troubles, reacting harshly when Wakana suggests she take a step back. The next day, as Naoko tries to get Konatsu to withdraw her application for use of the stage, Wakana explains what she learned by being able to sing with others, which reminds Naoko of Mahiru. Naoko decides to have the choir club participate in a commitee meeting to prove their growth. As the others send Sawa words of encouragement, she overhears her father as he pleas to the riding school to have them accept her. Encouraged by her friends, Sawa rushes on horseback to join her friends' sides at the commitee meeting.
After Taichi manages to get the photo of Sawa for the painter, he spots the props Wien made being taken away by the rubbish men. As the others head to the rubbish dump to recover them, Konatsu pleads with the student council to resume the school festival, only to have her proposal rejected. The next day, as Wakana visits Mahiru's grave, she runs into Naoko and shows her the song she and Mahiru wrote together. As Wein works on props, Taichi seeks help to make some costumes whilst Sawa pleads to the neighbourhood association in bringing an audience and Konatsu gets some help from the vocal group. Meanwhile, the higher ups plan to oppose anything delaying the construction.
As Wakana is approached by a strange foreigner on her way to school, Konatsu once again finds herself short on members as many of them suddenly quit, with the vice-principal threatening to dissolve the choir club, along with Taichi's badminton club, unless they can find sufficient members. Taichi manages to recruit Wien and takes him to his apartment, where he meets his sister. The next day, Taichi challenges Konatsu to a 2 vs 3 badminton match in order to obtain each other's members for their club, which inevitably results in the choir club's victory. As a compromise, Konatsu decides to make the club a joint choir and badminton club. As they once again go to the principal for his approval, he mentions a previous choir club which Wakana's mother was in. Although the vice-principal stands against the club, Taichi and Wakana help stand against her and the principal approves it. Upon hearing of a World Music Festival being held in the village, Konatsu decides to enter the choir club. Meanwhile, Wakana is once again approached by the strange foreigner.
Sawa brings Wakana and Konatsu to her house, showing Wakana what happened to Konatsu during last year's recital and getting her sign up for their new club in name. Putting their heads together, they manage to gather enough members to present to the principal, who approves it when he sees Wakana is involved and becomes its advisor. Afterwards, Konatsu gets Wakana to help her decide what kind of song they should practise, eventually settling on a song she used to hear her mother sing to her. However, the vice-principal is somewhat objective of the song for some reason. When the principal goes missing when the club needs to go to a joint rehearsel, Konatsu sends Wakana ahead of her whilst she waits for the principal. With the principal out due to a bicycle accident, Konatsu and Wakana get Ms. Takahashi to accompany them and arrive at the theatre, but the rest of the club is stuck in traffic. Not wanting to be deterred, Konatsu and Sawa go out on their own, with Wakana backing them up on piano, and manage to pull through. Afterwards, the vice-principal is put in charge of advising the club whilst the principal recovers.
On a school day, where a class gives flowers to their teacher, Ms. Takahashi, who is heading on maternity leave, a boy named Atsuhiro Maeda transfers into the class and is guided around the school by badminton player Taichi Tanaka. Meanwhile, a girl named Konatsu Miyamoto quits the choir club after becoming tired of not being able to sing and aspires to create a new choir club, of which she'll need five members. After managing to convince her best friend, Sawa Okita, to join and forcing her brother to apply as well, Konatsu tries to convince her other friend Wakana Sakai to join, but she doesn't want to sing for personal reasons. As Konatsu goes into town to practise her singing, her voice brings Wakana, Sawa, Taichi and Atsuhiro together to the same point.
As the gang show their support to Taichi as he participates in a bandminton tournament, Wakana appears to be occupied by the letter she received. The next day, Wakana runs into Sawa's mother, who shows her an old photo of Mahiru in high school with Naoko. Remembering how she had often argued with Mahiru over music before her death due to focusing on her exams, Wakana asks her father, who is to go on a short business trip, to get rid of the piano in her room. As Wakana returns home with the piano gone and realizing she is all alone, she recalls the contents of the letter, expressing Mahiru's desire to write songs with her, and soon starts to regret her decision.