Naota is a normal Japanese 6th grade boy (although a little cynical), but when his older brother leaves for America to play baseball, his brother leaves his 17 year old girlfriend Mamimi behind. Mamimi is sending mixed signals and advances to Naota, and he doesn’t know what to do about her. But to make matters worse, Naota’s world is totally turned upside down when he is run over by a woman on a Vespa. During their first encounter, she hits him over the head with her guitar, which then causes a horn to grow out of his forehead. She calls herself “Haruko” and her presence changes Naota’s life to even further insanity

The story of the film begins after the events of the second season, at the graduation ceremony of the third-year students. Knowing that the graduation of the third-year members means the end of μ’s, the nine girls take a walk around the school filled with memories. However, when the students attempt to leave the school gates, a single email is sent to them, and the curtain on a new story for μ’s begins.