Theo Asper is a 14-year-old orphan considered to be the weakest member of the mercenary guild. Viewed as nothing more than a liability, he is constantly dismissed from every party he joins. In his research to become stronger, Theo comes across information about a ritual that summons a wish-granting devil. Naively, he manages to summon Helvi with the plan of asking this devil for power. However, when he sees the angelic woman before him, his only wish is to marry her!

Charmed by Theo's cuteness and innocence, Helvi accepts his proposal. Although multiple people and monsters attempt to disrupt their relationship, Helvi's overwhelming strength easily overcomes any problem that stands in their way, leading her and the pure-hearted boy to a happy, newlywed life.