K-On!'s story revolves around four Japanese high school girls who join Sakuragaoka Girl's High School's light music club to try to save it from being disbanded, however, they are the only members of the club. At first Yui Hirasawa has no experience playing musical instruments or reading sheet music as she is only familiar with playing the castanets, but she eventually becomes an excellent guitar player. From then on, Yui, along with bassist Mio Akiyama, drummer Ritsu Tainaka, and keyboardist Tsumugi Kotobuki spend their school days practicing, performing, and hanging out together. The club is overseen by music teacher Sawako Yamanaka, who eventually becomes their homeroom teacher as well during their final year of high school. In their second year the club welcomes another guitarist, underclassman Azusa Nakano. After Azusa joins they gain more structure and begin to practice more.
After their third year, Yui, Ritsu, Mio and Tsumugi graduate and enroll into a university. There they join its light music club alongside three other students: Akira Wada, Ayame Yoshida, and Sachi Hayashi. Meanwhile, Azusa continues to run the high school light music club alongside Yui's sister, Ui, their classmate, Jun Suzuki, and new members Sumire Saitō and Nao Okuda.
K-On! (けいおん! Keion!?) is a Japanese four-panel comic strip seinen manga written and illustrated by Kakifly. The manga was serialized in Houbunsha's seinen manga magazine Manga Time Kirara between the May 2007 and October 2010 issues. It was also serialized in Houbunsha's magazine Manga Time Kirara Carat. The manga relaunched from April 2011 to June 2012 with two separate storylines published in Manga Time Kirara and Manga Time Kirara Carat. The manga is licensed in North America by Yen Press.
A 13-episode anime adaptation produced by Kyoto Animation aired in Japan between April and June 2009. An additional original video animation (OVA) episode was released in January 2010. A 26-episode second season, titled K-On!! (with two exclamation marks), aired in Japan between April and September 2010, with an OVA episode released in March 2011. Bandai Entertainment released the first season in North America in 2011, while the second season has been licensed by Sentai Filmworks. An anime film adaptation was released in Japan on December 3, 2011. The title of the series comes from the Japanese word for light music, keiongaku (軽音楽?), which in the Japanese context is similar to pop music.