During a winter long ago, near-immortal vampires began plaguing the world. As their population grew at an astounding rate, they promptly threw humanity off its place at the top of the food chain. Desperate to avoid extinction, the surviving humans concentrated in their last safe haven—a small city surrounded by an enormous wall. There, they have waged war against the vampires for many years. In the midst of a particularly brutal attack by the vampires, a human soldier named Momo encounters Fiine, the queen of their enemies. After seeing her try to save Momo's best friend from a rampaging vampire, Momo follows Fiine back to her house. It is there Momo connects with Fiine over their love for music and desire to escape the fighting. Pursuing that goal of escape, the two decide to journey toward a supposed paradise where humans and vampires coexist—despite being uncertain of its existence. However, Momo and Fiine are being targeted by both races, who they must first evade if they hope to reach their destination in one piece.

Originally from a big city, Futaba Ooki spends most days cataloguing her life through pictures on her phone. Upon moving to a seaside town, she gazes out over the vast expanse of sea and quickly becomes captivated by its endless possibilities. Unbeknownst to Futaba, her quiet contemplation catches the attention of local diving enthusiast Hikari Kohinata. Finding herself in the same class as Futaba the next day, Hikari is drawn to her quiet and shy demeanor and affectionately nicknames her Teko, sparking a tender friendship. Struggling to adapt to the change of pace in her life, Futaba finds herself strung along by Hikari's vast energy and passion for diving. Together, they join the school diving club, led by their homeroom teacher Mato Katori. With Hikari by her side, Futaba works to earn her diving certification while experiencing new friendships and possibilities.

The seven Blu-ray disc volumes of K-ON! have an extra short anime Ura-ON!

A two-episode special. They are designated as Episodes 13 and 14, and the first episode is a direct sequel to the previous series while the second episode is about the trans-sex of Natsuru.