Naota is an ordinary Japanese sixth-grader, but his life is complicated when his older brother departs for America to play baseball, leaving his 17-year-old girlfriend, Mamimi, behind. Mamimi sends mixed signals and makes advances toward Naota, leaving him unsure of how to handle the situation. Matters worsen when Naota's world is completely upended after being run over by a woman on a Vespa. During their initial encounter, she strikes him on the head with her guitar, causing a horn to sprout from his forehead. She introduces herself as "Haruko," and her presence plunges Naota's life into even greater insanity.