Harukaze Reito, a retired top-ranked adventurer, wants nothing more than a peaceful life.
But his quiet days are interrupted by the appearance of a broke and energetic gal-type girl, Hiiragi Karen, who asks:

"Hey, Onii-san! Will you treat me to a meal?"

Moved by her situation, Reito gives her a basic crash course in dungeon crawling.
Thanks to his guidance, she clears her first dungeon and excitedly declares:

"I did it, Master!"

Now, Reito’s dreams of a laid-back retirement start to crumble...