As a child, Kawagoe Haruma eagerly awaited the birth of his little sister. However, his mother had a miscarriage, followed by an operation that left her unable to bear children. That Christmas, Haruma made his Christmas wish. It was a wish so innocent and heartfelt that it was accepted instantly, unbeknownst to him.
Many years later, on Christmas morning, college student Haruma is awoken by a woman riding a hovering motorcycle and claiming to be Santa Claus. She throws him a bag, and he opens it to find his new little sister!
Complaining to Santa Claus that he made this wish a long time ago, she tells him that it takes much longer for a sister than it does a toy car. After naming her Choko because of the Anchoko (mini-guidebook) she carries around, they begin their new life as brother and sister in a crazy apartment building filled with interesting neighbors.