On his way home from school, Jun rescues a flying squirrel fleeing from a flock of crows. Kept as a pet since it was a baby, the flying squirrel can't fly nor can it even find food for itself. Jun feels sorry and decides to keep it, naming it "Donguri" (Acorn).

While taking part in the clean up of the digital camera clubroom, Nao, Miki and Jun finds a cardboard box on the top shelf. There is an old album in the box filled only with photos of clouds. Nao grins when Miki says, "Here is your ancestor." But her smile disappears when she opens the next page. It was a photo of a female student holding a baby in one arm and pressing the shutter of a small camera with the other hand. In order to return the album to the owner, the threesome, forgetting that they were cleaning the room, start their search with a list of club members as their only clue and the date "September 9, 1991" marked on the back of the album.