Due to her mother's work, Chizuko Date has never remained in one place for long or grown attached to anything in particular. However, while temporarily staying with her grandfather Suikyou, she witnesses something extraordinary: a magical girl named Berry Blossom fighting against Chroma, member of the evil organization Fossa Magna. Seeing Berry Blossom save the day ignites a passion in Chizuko, and she swears to be a fan of the new magical girl in town thereafter.
As time passes, Berry Blossom's popularity dwindles due to the incompetence of her opponent. When Chizuko randomly encounters Chroma himself, she does not hold back her complaints against him. For his part, Chroma does not enjoy being defeated again and again; moreover, Fossa Magna will force him to resign if he continues losing. In desperation, Chroma begs Chizuko to become his second-in-command and help him with his evildoings. For the sake of Berry Blossom regaining the spotlight, Chizuko must question whether it is right to join forces with the magical girl's sworn enemy.