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 barren. That Christmas, Haruma made his Christmas wish. It was an innocent and heartfelt wish 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.