The story centers around a Christmas wish made by a young Haruma Kawagoe, who was eagerly anticipating having a baby sister, after his mother suffered a miscarriage followed by a hysterectomy.

Several years later, when Haruma is a college student, a woman on a flying motorbike claiming to be Santa Claus delivers his wish, a younger sister.

When she asks him to name her, he calls her Choko, which refers to anchoko (あんちょこ), the Japanese word for "cheat sheet" or "study guide".

[1] Written by Gō Zappa and illustrated by Sakura Takeuchi, Chocotto Sister was serialized in Hakusensha's seinen manga magazine Young Animal from 2003 to 2007.

[2] Chocotto Sister was produced by Nomad, directed by Yasuhiro Kuroda, and written by original creator Gō Zappa, with music by Masara Nishida and character designs by Yukihiro Kitano.