A boy falls in love with a magical girl.
Unable to confess to her since she's a public figure and a whole generation older than him, he simply relegates to being her number one fan. Tragedy strikes him when it becomes apparent that the other little boys in his class don't think highly of her since it's not boy thing to like so peer pressure makes him hide away his love for her in secret and he joins the others in jeering at the magical girl's son who happens to be his classmate.
The son transfers out and the boy comes to regret in joining in the teasing and tries to put it behind him but as he grows up, and try as he may, he finds it harder and harder to deny his passion and admiration towards magical girls. By the time he would become a middle schooler he finds himself at a crossroads where he must either forsake his guilty pleasure or continue to suffer living a very repressed life. The solution to his problem however soon literally falls into his lap.
The evil organization that his hero fights is making a revival and one of their out of luck and hungry dark familiars finds itself in a desperate pinch to recruit a contractor into its organization lest he runs out of magic power and disappears forever. Sensing the boy's affinity for magical girls and uncovering his repression, it offers him a deal - become a warrior for their forces and get the chance deal directly with his hero. It takes some convincing but he accepts and they enter the contract. There was a catch however - he would be fighting for them as a dark magical girl.
It takes some getting used to and the boy spends most of his middle school stay training up for the big plan but he comes to embrace the role and lets this new feminine element of his alter ego seep into his regular life. However, when he learns he'd be reuniting with his hero's son in high school, he goes all in and makes a complete image change - for both himself and his cause.