The story begins with the protagonist, Ars Krate, awakening in a dark, metallic coffin-like chamber. He finds himself naked and disoriented, with a throbbing headache that intensifies when he tries to recall his past. The chamber contains twenty such coffins, and upon opening his, he finds clothes, food, and water. After consuming some of the provisions, Ars ventures out, following an arrow painted on the wall. He is immediately confronted by a reptilian humanoid, a Lizardman, which he narrowly escapes by throwing a chocolate bar to distract it and sliding through a closing door.
He finds himself in a control room, where he activates the lights, revealing a sophisticated setup with monitors and consoles. He discovers a "Control Key" for the "Olympus Church, New Titan District Shelter." His attempts to remember his past are thwarted by painful headaches. He also finds a note from his former teacher, Meier Nine, apologizing, suggesting she may have placed him there. Ars checks his belongings: a smartphone (dead battery), wallet with some money and his student ID, and a large amount of flour. He realizes his current food supply is limited but deduces that the other twenty coffins might contain similar provisions, offering a potential eighteen days of sustenance. However, the presence of the Lizardman in the hallway prevents him from retrieving them.
A crucial turning point occurs when Ars receives an email titled "To Those Who Survived" from "Pale Operator." The email reveals that the world has been devastated by a nuclear war and merged with another dimension 100 years prior. It introduces the concept of "Maso" (magic energy) that permeates the world, which can cause people to become "Demon-humans" if not controlled. The email prompts Ars to download three applications: "Adept System" to manage Maso, a "Monster Encyclopedia," and "Post-Collapse Historical Records."
The Adept System allows Ars to gain power by defeating monsters, which he learns are prevalent in this new world. He discovers that the shelter he is in is now a "Dungeon," and the Lizardman is a monster. He learns to operate the shelter's security systems, including gun turrets. Ars successfully defeats the first Lizardman using a gun turret and absorbs the resulting purple mist, which he identifies as Maso, gaining experience and leveling up. He also finds dropped items, including scales.
Ars then ventures out to gather more supplies from the cryo-sleep pods, collecting food and clothing. He also finds a heavy sword and shield from a defeated "Lizard Knight," a stronger variant of the Lizardman. He acquires a robotic dog named Pochi, which becomes his loyal companion. Ars faces more Lizardmen and Lizard Knights, using the gun turrets and his growing combat skills. He also discovers heavy security robots, the JL601s, in a guarded room.
After several days of fighting monsters and gathering resources, Ars decides to confront the Dungeon Core, the source of the monsters. He uses the heavy security robots to fight the Dungeon Boss, a giant spider called the Dark Metal Spider, and its offspring. The battle is intense, resulting in the destruction of two robots and the damage of another. Ars uses a desperate tactic involving the robots' flamethrowers and a sprinkler system to defeat the spider.
Following the boss's defeat, Ars absorbs a massive amount of Maso, leveling up significantly. He then proceeds to the power room to confront the Dungeon Core. He is overwhelmed by the cold and the core's influence but manages to resist by channeling his internalized Maso. Using his bar, he shatters the Dungeon Core. This triggers a massive influx of Maso, causing an error in the Adept System. A mysterious blue figure appears and seemingly stabilizes Ars before he loses consciousness.
Upon waking, Ars finds Pochi by his side and receives a notification from the Adept System: he must undergo a class change within three hours to prevent the absorbed Maso from going berserk. He is presented with three potential classes: Technoshaman, Psychic Wizard, and Regenerate Soldier.
