Momoko Futo is an average high-school girl going about her everyday life. Though laid-back and cheerful, her life is anything but mundane as her eccentricity and clumsiness never fail to spice up her days. Her two best friends are always with her: the cute and innocent Mayumi Furui, and the calm and cool Shibumi Shibusawa. Without any spoken dialogue or narration, Joshikausei aims to recount the comedic shenanigans these girls get up to through the expressive sounds and gestures that they make.

As she heads off to her entrance ceremony at Sainotama Girls' High School, Akane Kikuchi muses over her grade school dream of becoming a manga artist and the lack of progress that she has made. When she finally arrives at school, she is surprised to learn that she is once again in the same class as her two best friends: the deadpan and emotionless Shiori Saginomiya and the hyperactive and ridiculous Nozomu Tanaka. Tanaka then comes to the obvious realization that she can't achieve her grade school dream of being popular with the boys and getting a boyfriend by going to an all-girls high school. In desperation, she begins asking the girls in her class to introduce her to their guy friends. Her classmates, however, are anything but ordinary. From a grandmother-loving loli to a reclusive chuunibyou to an overly analytical stalker, each one is given a fitting nickname by Tanaka to accentuate their weirdness. And so begin the wasteful days of these high school girls, each day kicked off with a simple question: Hey, wanna hear something amazing?

It is the new year, which means that the senior members of the Light Music Club are now third-years, with Azusa Nakano being the only second-year. The seniors soon realize that Azusa will be the only member left once they graduate and decide to recruit new members. Despite trying many methods of attracting underclassmen—handing out fliers, bringing people into the clubroom, and performing at the welcoming ceremony—there are no signs of anyone that plans to join. While heading to the clubroom, Azusa overhears Yui Hirasawa say that the club is fine with only five people and that they can do many fun things together. Changing her mind, she decides that they do not need to recruit any members for the time being. K-On!! revolves around the members of the Light Music Club as they experience their daily high school life. From rehearsing for concerts to just messing around, they are ready to make their last year together an exciting one!

A fresh high school year always means much to come, and one of those things is joining a club. Being in a dilemma about which club to join, Yui Hirasawa stumbles upon and applies for the Light Music Club, which she misinterprets to be about playing simple instruments, such as castanets. Unable to play an instrument, she decides to visit to apologize and quit. Meanwhile, the Light Music Club faces disbandment due to a lack of members. This causes the club members to offer anything, from food to slacking off during club time, in order to convince Yui to join. Despite their efforts, Yui insists on leaving due to her lack of musical experience. As a last resort, they play a piece for Yui, which sparks her fiery passion and finally convinces her to join the club. From then onward, it is just plain messing around with bits and pieces of practice. The members of the Light Music Club are ready to make their time together a delightful one!