|Ritsu and Mio attended the same elementary school in the past and were classmates. Ritsu liked Mio for certain reasons and started to continiously pick on her. One day, she found her depressed on a playground. Mio told her that a composition she wrote at school won an award and she had to read it in front of a large audience. Because Mio was shy in nature, Ritsu helped her train for the reading. She taught her to imagine the audience as pineapples to stay calm which actually helped Mio to read her text without problems. After that, Mio continued to visit Ritsu's home and the two became best friends. Miraculously, the two were always in the same school and class together.
Years later, Ritsu and Mio watched a concert at TV which amazed Ritsu so much that much that she convinced Mio to form a band together. After buying the sticks at first, she needed to save a lot of money to be able to finally buy a used drum kit.
After graduating from Junior High School, she was accepted into Sakuragaoka High School.
Instantly after starting her high school career, Ritsu wanted to join the school's Light Music Club. She forced Mio to join her by destroying Mio's application to join the Literature Club, and then asked the teacher Sawako Yamanaka about the whereabouts of the club. After hearing that all members graduated, she came to the conclusion that she would be the club's next president if she joins it first. Thereafter, she officially starts the club with her and Mio as the first two club members and being the club's president.
Deciding to wait for new members to arrive, Ritsu is a bit disappointed to notice that no students are interested in joining the Light Music Club, but after that, she and Mio meet a student, Tsumugi Kotobuki, who was looking for the Choir Club, at which Ritsu immediately begs her to join the club, resulting in Mio hitting her over the head. When Mio decides to leave to continue on giving her club application to the Literature Club, she tells Mio about a promise they made to form a band some day after watching a live concert together, before her memory of that time is corrected by Mio herself. To her joy, Mugi joins the club after changing her mind.
Ritsu then holds a club meeting with Mio and Mugi at a fast food restaurant to discuss about getting another member to save the club from being permanently disbanded, with most of the ideas they come with proving to be bad. The next day, she, Mio and Mugi decide to make leaflets in order to attract other students' attention to the club in the hopes of getting a new member. She then gets an application form about a girl named Yui Hirasawa, whom she then imagines is a prodigy in playing the guitar. Unfortunately, when she and the rest finally meet her, they find out that Yui does not actually know how to play any instrument but holds a small show for her, with Yui finally joining the club with her gladly accepting her in.
Upon learning about the Light Music Club, Ritsu Tainaka goes off in search of her childhood best friend, Mio Akiyama, and finds her in the hallways of their new high school. When she learns that Mio is about to join the literature club, she takes a look at her club application form and suddenly rips it in half before she drags Mio along with her to the school faculty.
Unfortunately, Ritsu learns from one of the teachers that the club is about to be disbanded due to the previous members having already graduated last year but informs her that the club needs at least four new members to have it stay intact. Having an idea about starting the club, Ritsu then asks a girl that is looking around to join their club but is stopped by Mio due to this. When Mio decides to leave, Ritsu calls her out for backing out on a promise they supposedly made, but Mio corrects her for this ruse. The girl then decides to join their club nevertheless, expressing how fun it might be, thus signalling the start of the Light Music Club's beginning but now have to look for one more member to officially start.Some time later, Ritsu arrives at their club room and meets a girl hanging outside and immediately learns that she is Yui Hirasawa, their much awaited new member of the club. Ritsu immediately introduces Yui to the other two members; Mio and Tsumugi, ordering Mugi to prepare some tea for Yui. Despite her excitement to have met her at least, Ritsu is saddened to learn that Yui was planning to quit the club which is why she went over. Ritsu, however, proposes that she and the other members play a piece of music they had just whipped up to her and Yui accepts. Although given brutal honesty by Yui that their song "wasn't all that good", it is enough to convince Yui to join the Light Music Club, which Ritsu happily welcomes her to be a member of.
The next day, when Yui drops by the club after class, Ritsu is asked by her why she chose the drums. Ritsu explains that she finds playing instruments like the guitar and keyboard make her anxious because of how much a person has to deal with using their fingers on playing them. Soon after, Ritsu proposes that their first order of business as an official club be practice. However, Yui still doesn't have an instrument to play and Ritsu playfully rejects Yui's attempt borrow money from her to buy a guitar. On Mugi's idea, Ritsu has the group meet the very next day to visit a music store to help Yui pick a guitar for herself. Immediately the next day, Ritsu arrives with Mio and Mugi while waiting for Yui who shows up a bit late. While on the way to the music store, Ritsu stops Yui from getting too distracted from what they are really there for. Eventually when they reach the music store, Yui has already found a guitar that she becomes attracted to, but they find it too expensive for her to buy. Fortunately, Mugi comes with great news that the shop employees will sell the guitar at an affordable price for Yui, which Ritsu immediately asks her how she managed to have the staff do so.The next day, Ritsu is along with Mugi and Mio when Yui shows off her new guitar to them. When she asks Yui to play a chord for them, she gets immediately disappointed as Yui still doesn't know how to play her new guitar yet. After finding out that Yui hasn't taken the film off her guitar, Ritsu immediately does it herself, causing Yui to get upset. Ritsu apologizes and treats Yui to some of the sweets Mugi brought, which instantly lightens up Yui. A moment later, after Yui notes how scary guitar strings are, Ritsu tells her to be careful of handling her guitar as the strings might cut her fingers, a comment which frightens Mio.
The very next day, after the exams, Ritsu loudly announces that she finally finished her exam. She and the others learn that Yui didn't pass her test and, along with Mugi, encourage Yui to do her best on her make-up exam, but comically takes it back when Yui says that she didn't study at all. Yui then asks to see her test paper, and Ritsu proudly shows her average score, which Yui comments that her scores don't look true to her actual character, much to Ritsu's chagrin. Ritsu then tries to say that she studied very well, until Mio interjects and reveals that she had helped Ritsu study for it. Upon hearing that Yui believes that the club won't be any good, Ritsu asks her why and upon hearing Yui say that she will just come by and eat sweets, Ritsu momentarily accepts, before taking back her words, putting Yui on a choke hold and telling her to stop kidding around. Upon hearing that Mio will help Yui study for her make-up exam, Ritsu helps encourage the two, much to Mio's embarrassment. Arriving at the Hirasawa household, Ritsu and the others are greeted by Yui's younger sister, Ui Hirasawa, whom the three note is more mature than Yui, which slightly offends Yui.
While Mio helps Yui study, Ritsu gets bored and plays around the room making all sorts of ruckus, earning her a lump on the head from an annoyed Mio. She then messes with Yui as she studies, receiving another lump on the head. The next day, Ritsu and the others celebrate Yui's perfect score from her make-up exam.