In the Karearondo Empire, a rumor spreads through the capital about a mysterious handsome young man who solves problems for anyone who drops a letter into a certain request box. This young man is Rote Grantle, a genius mage who wields the world's strongest defensive magic and works as the personal butler and bodyguard to the third princess, Kurel Karearondo.

Kurel possesses tremendous magical power that makes even her own allies fearful, but her everyday personality is that of a clumsy, accident-prone princess who is easily flustered by affection. Rote is impeccably competent at his duties, handling all manner of household tasks perfectly while also protecting Kurel with his supreme defensive abilities. He teases her relentlessly with sharp-tongued remarks yet openly declares his love for her without hesitation, and Kurel, for her part, trusts and loves him completely despite blushing at his constant provocations.

The story begins when the pair receives a request from a national magical museum to protect a national treasure known as the magic key Etoya, an artifact that serves as a partner to a mage through a contractual bond. The magic key belongs to a ranking system of seven tiers, and this particular key is among the highest ranks. A warning letter has announced an intention to steal it. As Rote and Kurel investigate, they meet museum director Strum and staff member Erune, learning unfortunately that reinforcements from the palace are unlikely.

While dining at the restaurant of an acquaintance during their investigation, Kurel is poisoned by a deadly substance slipped into her food. This event enrages Rote and escalates his vigilance dramatically. The true culprit behind the theft and poisoning is revealed to harbor a deep sorrow over having a loved one taken by the corrupt empire, directing that hatred toward the nation itself and targeting Kurel specifically. Rote finds he can understand that love which drives the villain, even if he cannot sympathize with their methods. A massive magical attack threatening to destroy the entire capital forces Rote and Kurel to combine their unique powers something only this pair can achieve to counter the devastation while fulfilling the princesses wishes.

The series concludes as the beloved relationship between the third princess and her almighty butler drives the final battle to its resolution.