The story begins with Honjou Mamori, a hikikomori who works remotely and lives a reclusive life due to severe social phobia. Her parents, though understanding of her condition, constantly try to "cure" her, which Mamori finds torturous. Her hikikomori status stems from her extreme beauty, which attracted unwanted attention, including stalking, harassment, and even a near-rape during her school days. This trauma led her to withdraw completely from society.
One night, Mamori's peaceful existence is shattered when a man breaks into her house. He is a former stalker from her school days, who, in his deranged state, confesses to murdering her parents. He then attacks Mamori, but in a desperate act of self-defense, she kills him with a kitchen knife. Overwhelmed by grief and shock, Mamori collapses. Upon waking, she finds herself in a cave, realizing she has been transported to another world.
She discovers she has become a "Dungeon Master" after touching a "Dungeon Core," a crystal that serves as the heart of a dungeon. This new role binds her to the dungeon's growth and defense. Initially, Mamori feels no will to live, having lost her parents and her only sanctuary. However, when three goblins attack, her survival instinct kicks in. Using her newfound abilities, she summons a "Living Armor" to defend her. The battle is fierce, but the Living Armor, with Mamori's strategic use of traps and its own strength, defeats the goblins.
Mamori then begins to fortify her dungeon, prioritizing defense and concealment. She upgrades the Living Armor, sets traps, and creates a maze-like structure to deter intruders, aiming to create an impenetrable sanctuary where she can continue her hikikomori lifestyle undisturbed. She also discovers she has gained significant power, including an unusually high MP pool and the unique skill "Great Mage," which explains her ability to summon powerful beings.
Meanwhile, in a classroom on Earth, a group of students, including the popular idol Honjou Mamori, are suddenly engulfed by a magical light. The narrator, a keen observer of human behavior, realizes they are being summoned to another world. This group includes Mamori's former classmates: the handsome and idealistic Shindo Yuma, the intelligent and kind-hearted magician Adel, and the skilled swordsman Keith. They were on a goblin extermination quest when they were unexpectedly transported.
The narrative then shifts back to Mamori's dungeon. She continues to develop her defenses, creating a labyrinthine structure that appears as a natural cave to outsiders. Her goal is to make her dungeon an unwelcoming place for any potential invaders, ensuring her safety and solitude. She reflects on her past trauma and her desire to live a life free from fear and interaction.
Later, the summoned group of students, now in the dungeon, are ambushed by the Living Armor. Keith is killed instantly, and Adel is soon after. Doyle, the last survivor, is overwhelmed by the darkness and the realization of his impending death, lamenting their fate.
Mamori, observing the battle from her core room, notes the adventurers' stats and the effectiveness of her defenses. She realizes that her dungeon's deceptive appearance as a mere cave was a key factor in their downfall. She gains experience and DP from their defeat, further strengthening her dungeon and her own abilities. She decides to continue developing her dungeon, aiming to create a perfect, inescapable sanctuary where she can finally achieve her ideal hikikomori existence, vowing to eliminate anyone who threatens her peace.
