When a young boy learns he has a new older sister after his father remarries, he is thrilled at first. However, his excitement fades when he discovers that his new sister is none other than his school's stern physical education instructor, Shina. As his father moves in with his new spouse, the boy is left to live alone with Shina. To his surprise, he soon realizes that the Shina he knows at home is a far cry from the strict teacher he's used to—she is clumsy, afraid of bugs, and surprisingly kind.