Kizaki Touka, a pragmatic young man, and his cheerful, sharp-tongued younger sister, Haruka, are introduced as seasoned dungeon explorers, effortlessly defeating a formidable Minotaur-like boss on the 15th floor. Touka deflects its axe with daggers while Haruka's explosive magic and binding spells create an opening for Touka to deliver the finishing blow with a scythe. This initial scene establishes their combat prowess and the existence of dungeons, magic, and monster drops, though Touka laments the boss's "lame" rewards.
The narrative then shifts to the '081 Disaster,' a global event where dungeons suddenly appeared on Earth. Modern weapons proved useless inside, leading to the discovery of unique dungeon rules: no electricity, strength gained from primitive combat (which diminishes outside), and a class system. Touka and Haruka's personal tragedy unfolds: their mother died in a drunk-driving accident, and their father abandoned them, taking their savings and leaving them impoverished. They moved into a dilapidated former warehouse, with Touka working odd jobs and Haruka using her crafting skills to make repairs.
Their precarious life is upended when a dungeon entrance unexpectedly appears in their basement. Fearing eviction if they report it, they decide to explore it themselves, hoping to find resources. After observing a resilient slime, they equip themselves with a sturdy gate for their basement, and improvised weapons like a triangular hoe for Touka and a crowbar for Haruka.
Their first dungeon dive reveals the class system, with Touka choosing 'Enchant' and Haruka 'Magic,' a suboptimal combination for direct combat. Haruka bravely steps up as a frontline fighter, while Touka supports with 'Speed' enchantment. They struggle initially against slimes, realizing the monsters are tougher than expected and that magic consumes an internal resource (MP). Despite the challenges and lack of item drops, they persist, developing effective combat styles and leveling up. They quickly clear the first four floors, even encountering a rare "experience rabbit" that grants significant growth.
On the fifth floor, they face a Hobgoblin boss. Their combined tactics of blinding it with magic and overwhelming it with physical attacks lead to a brutal victory. However, the sheer violence of the kill deeply disturbs them, leaving them mentally exhausted. Seeking a "refresh," they visit a game center, where they use their dungeon-honed senses to win snacks like beef jerky for future provisions.
Haruka, inspired, begins training with Touka, and they establish a rigorous pre-dungeon routine. Returning to the dungeon, they find themselves significantly stronger, easily dispatching lower-floor monsters. On the fifth floor, they discover a teleportation magic circle, which, after defeating the boss, transports them to a new, deeper area. Here, they encounter new wolf-like monsters that utilize "skills" for enhanced combat. This marks a turning point as these monsters finally yield item drops: wolf claws and mysterious stones. Soon after, they encounter a pig-like monster, which, after a chaotic fight, drops a coveted piece of meat.
Overjoyed by this valuable food source, they embark on a "steak hunt," accumulating a significant haul of monster parts and meat. They discover another magic circle that allows them to teleport back to a newly formed "transfer room" on the first floor. Back home, they cook the monster meat, finding it incredibly delicious and invigorating. This newfound source of sustenance and the thrill of exploration transform their grim existence. They commit to continued dungeon exploration, driven by the goal of "rising up" from poverty and securing a better future.
