The story begins with the protagonist experiencing an ordinary summer evening, which is abruptly shattered by the appearance of approximately fifty individuals clad in hooded cloaks, resembling "teru teru bozu" (Japanese rain dolls). These figures declare their intention to extract the "world's core" and proceed to draw a magic circle on the asphalt. The leader, a prominent figure among them, plunges his hand into the ground and retrieves a tear-shaped blue gem, which they refer to as the "world's core."
Immediately, a violent earthquake strikes, revealing that the cloaked figures are levitating a few centimeters above the ground. They discuss their actions, mentioning that the world is constructed from magic and information, and that without the core, everything will dissipate into magic. They express satisfaction with the "magic collection" and their ability to defeat "Grandem." As the world begins to crumble and dissolve, people and objects disappear. The protagonist, witnessing this destruction and the indifference of the cloaked figures, instinctively kicks off the collapsing ground and approaches them. She manages to land a flying kick on the leader, knocking him down and retrieving the "world's core."
Attempting to use the core herself by plunging her hand into the ground, the protagonist finds it ineffective. She flees as the cloaked figures pursue her, with one named Offle being particularly persistent. She eventually reaches a stable patch of ground and places her hand holding the core on it, only for the ground to vanish, sending her falling into an abyss. Unlike others who disappear, she continues to fall, clutching the core, as her world disintegrates around her.
She awakens in a stone room illuminated by a faint blue light, featuring a magic circle, tapestries, a large hourglass, and a throne. Offle, the pursuing cloaked figure, is also present. He demands the core back, and when the protagonist refuses, he draws a knife, intending to kill her. Realizing she is unarmed and trapped, she uses her agility to trip him, causing him to fatally stab himself with his own knife.
After confirming Offle's death, the protagonist explores the room for an exit but finds none. Overwhelmed by despair and the loss of her world and loved ones, she decides to rest on the throne. Upon sitting, she is flooded with information and undergoes a transformation, becoming a "Dungeon." She learns she can now use souls to create monsters, traps, and modify the dungeon, and that the "world's core" might allow her to recreate her original world if she gathers enough souls.
Her initial soul count is a meager 12 points, derived from Offle's death and the dungeon's activation. She discovers the dungeon consists of three 12-tatami mat rooms connected by corridors, leading to the throne room. She also salvages items from Offle's body: a cloak, a robe, a staff with a magic stone, a knife, medicinal herbs, and candy. The corpse itself is unusable for creating monsters due to a "holy seal."
The protagonist then faces her first challenge: three intruders enter the dungeon. With limited resources, she devises a plan to lure them into traps and divide them. She modifies the doors, adds a numerical lock, and places a mathematical puzzle on the wall as a diversion. She also prepares a shovel as her primary weapon and uses the magic stone from Offle's staff.
The intruders, a group of adventurers led by Hemblen, are initially disappointed by the dungeon's apparent lack of treasure. They are split by a trap, with Hemblen, Jas, and Kev proceeding through a maze-like section, while Misa goes through a straightforward path. Kev solves the initial puzzle, but the numerical lock proves to be a red herring. Jas breaks down the door, and Misa is separated by a trap. Kev and Jas enter a maze, where Kev is killed by a trap. Hemblen and Jas proceed, eventually encountering the protagonist, who uses her abilities to defeat them.
The protagonist gains a significant amount of soul points and items from the defeated adventurers. She levels up to Lv3 and acquires the "One-Point Breakthrough" skill. She then uses her resources to enhance herself and her dungeon, creating a "Living Armor" monster and upgrading her own equipment. She also establishes a farm within the dungeon and creates a stable for a horse, obtained from the defeated adventurers.
Further intrusions occur, including a powerful swordsman who defeats the Living Armor. The protagonist, now Lv5 and equipped with the "One-Point Breakthrough" skill, manages to defeat him through a combination of strategy and her enhanced abilities. She then faces a group of five adventurers, including Hemblen, Kev, and Jas, whom she defeats using her traps and the Living Armor. She gains a substantial amount of soul points and items, including a map of the surrounding region.
The protagonist continues to strengthen her dungeon and monsters, creating new soul-based monsters that can be equipped by herself or the Living Armor. She also expands the dungeon by adding new floors and elaborate traps designed to divide and conquer intruders. The narrative culminates with the protagonist facing a new group of five adventurers, preparing to defend her dungeon and continue her quest to reclaim her lost world.
