A boy falls in love with a girl.
Having had his confession rejected by her on the basis that he's not grand enough for her, he seeks out ways to improve himself as one. Fate intervenes in the form of a badly weakened magical girl that had just come from a fight in the magical realm. Not recognizing her as his long separated fiery childhood friend he accepts a pact with her to help her out as her familiar.
Something goes very wrong though as instead of finding himself empowered by her magic, he and the girl swap bodies. Quickly adjusting to this odd predicament, they discover that while their contract did not turn out how they predicted he somehow possesses aptitudes for magic far beyond the girl herself.
Making quick work of the beasts from the other world, he undoes the magical girl transformation and they both return back to their respective bodies. They get reacquainted with each other and she explains that she's one of several chosen human representatives by warring factions in the other world. The demon tribe has been on the rise with the goal of the unification of both worlds under their great demoness leader and they are just steamrolling all the other races.
After a few bouts with rival human world agents, things take a turn for the ridiculous when a fated encounter reveals the great demoness leader is in fact the girl the MC fancies and that she's taken by his actions in the magical girl's body and marks the magical girl as her top pick for her human world spouse.
Faced with making the right decision for the future of their worlds, MC must buy time by engaging in a farcical love polygon of epic shakespearean proportions and triumph over all the challenges the girl he loves sends his way to judge his worth.