The story begins with Keita Suminoe, a male third-year junior-high school student studying for his high school entrance exams. He is living in a home with his older twin stepsisters, Ako and Riko, who share no blood relation to him, and they help him prepare for his exams. Even though he initially dislikes himself for it, he begins to become attracted to his two step-sisters, and his two parents encourage him to eventually get married to one of them.

Mahoromatic is a sci-fi romantic comedy manga and anime series which contains elements of the literary genre of tragic dramas. It is about a female android former soldier, Mahoro. Driven by guilt from her actions during her combat days, she decides to dedicate the rest of her life to serving the son of her late commander as a maid.

Subaru is a young girl who discovers that her best friend Aoi is part of magical group trying to gather engine fragments so that a little being from Pleiades can return to his home. Subaru considers joining them as she was chosen by the Pleiadian, president of this afterschool club.