Death is one of many mysteries that has left humanity in the dark since the dawn of time. However, the burning question of what happens to the soul after one dies is soon answered to 17-year-old Yuzuru Otonashi. Waking up with no previous memories in a dimension between life and death, he discovers the unsettling truth of the afterlife. Taking the form of a high school, this bizarre dimension is designated to shelter those who died unwanted deaths. Feeling wronged by God during their earthly lives, the school's residents have decided to form the Afterlife Battlefront—a rebellious faction determined to oppose their god-like student council president, Kanade Angel Tachibana. The group's leader, Yuri Nakamura, recruits Otonashi in their fight against Angel in order to take control of their own lives. However, questioning the morality behind their actions, Otonashi takes a step behind the enemy lines to understand the opposing side of their common fate.

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!

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!

Yearning to make friends and perform live with a band, lonely and socially anxious Hitori Bocchi Gotou devotes her time to playing the guitar. On a fateful day, Bocchi meets the outgoing drummer Nijika Ijichi, who invites her to join Kessoku Band when their guitarist, Ikuyo Kita, flees before their first show. Soon after, Bocchi meets her final bandmate—the cool bassist Ryou Yamada. Although their first performance together is subpar, the girls feel empowered by their shared love for music, and they are soon rejoined by Kita. Finding happiness in performing, Bocchi and her bandmates put their hearts into improving as musicians while making the most of their fleeting high school days.