Run Momoki, who is forgetful yet good-natured, has been best friends with Tooru Ichii since childhood. Even though Run is a year older, her carefree attitude and popularity among boys constantly frustrate the overprotective Tooru. To make things more complicated, when Tooru applies to the same high school as Run, she discovers that their duo has expanded into a quartet. Their new acquaintances, Nagisa Tennouji and Yuuko Nishi, are Run's classmates and, in Tooru's view, must prove their trustworthiness. As the new high school year begins, the girls balance their friendship with the exciting opportunities of school life.