Mercenary adventurer Theo Asper is a fourteen-year-old orphan who struggles to survive in a fantasy world. Abandoned as a baby, he was taken in by a loving elderly couple, but they died when he turned twelve, leaving him completely alone. Determined to become strong enough to live independently, Theo joins the mercenary guild, but he has no magic or notable physical ability. He is repeatedly kicked out of parties who see him as a burden and only hire him to carry gear. Desperate for a solution, Theo researches ancient texts and discovers a ritual to summon a wish‑granting devil.

Theo successfully summons the devil Helvi, a stunningly beautiful woman who has not been called in many years. Helvi offers to grant any wish in exchange for something precious to him. Overwhelmed by her appearance, Theo completely forgets his original desire for power and blurts out a marriage proposal. Moved by his pure, earnest sincerity, Helvi accepts, and they instantly become husband and wife.

From that moment, the story follows the newlyweds as they navigate their unconventional life together. Helvi returns to the guild with Theo and confronts the party that mistreated him, effortlessly demonstrating her overwhelming strength. She then accompanies him on quests, easily dispatching monsters and problems that once seemed insurmountable. Despite her immense power, Helvi is wholeheartedly devoted to her young husband, doting on him and often teasing him. Theo, in turn, is kind‑hearted and humble, adjusting to the sudden change in his fortunes.

The narrative is episodic, mixing comedy, romance, and fantasy slice‑of‑life elements. Theo and Helvi encounter a cast of recurring characters, including the gentle receptionist Fiore, the boisterous adventurer Lauro, and the flirtatious noble Seria, who each react to the unusual couple in their own ways. The series also introduces minor antagonists—people and monsters who try to interfere with their relationship—but Helvi’s power makes short work of them. Beyond the guild, the couple explores the world, takes on odd jobs, and gradually deepens their bond.

The setting is a generic medieval fantasy world with magic, monsters, and adventurers’ guilds. Notable narrative arcs involve Theo’s lingering insecurity about his own weakness, Helvi’s mysterious past as a devil, and their interactions with other adventurers who initially mock or underestimate them. Over time, the couple’s genuine affection and Helvi’s sheer capability earn respect from those around them. The story maintains a light, wholesome tone, though it occasionally includes suggestive humor and mild fan service. Overall, it focuses on the charm of an unlikely romantic pair and the daily adventures that come with having an all‑powerful devil wife.