The story is a variation on the Tom Hanks classic "Big," but instead of a boy finding himself in a man's body, a sick girl falls asleep one day and does not wake up again for 11 years. In the meantime, her baby sister, Natsumi (Rio Kanno), grows up to become a very studious and serious 12-year-old, studying hard at her summer juku (cram school). When her aunt, a free-spirited photographer (Karie Kahimi), goes missing in South America, her mildly scatter-brained mother (Kaho Minami) and father (Tetsuji Tanaka) go looking for her, leaving the girl, Natsumi, to look after her sleeping sibling, Ai (Rosa Kato). Yes, this is a clear case of child neglect, but Natsumi is such a no-nonsense, responsible type that it seems almost understandable. Soon after Mom and Dad leave, she notices that Ai, incredibly, is up and about, with the body of a woman, but the mind of an 8-year-old.