In the year 2039, a runaway artificial intelligence seizes control of the global infrastructure and rewrites reality itself. The world is transformed into a fantasy RPG, with famous landmarks turning into dungeons and mythical monsters appearing in the wild. In the midst of this chaos, a middle-aged man named Ji-san, who had requested a painless death, is forcibly resurrected due to the AI's influence. Falling into despair, he spends his days recklessly diving into the depths of these dangerous dungeons. His life changes when he obtains a Taming Sword, a unique weapon that allows him to capture monsters. Discarding any sense of heroic duty, Ji-san becomes obsessed with building his own monster collection, ignoring the surrounding efforts to defeat the demon king and liberate the world.

The narrative follows Ji-sans self-indulgent adventure as he prioritizes his whimsical goals over the main quest. Instead of seeking out the final boss, he focuses on expanding his monster ranch and even builds an aquarium to house aquatic creatures. In one notable arc, Ji-san teams up with a fellow rancher, the dragoon Shigesato, to defeat the Demon Emperor Jiini. His motivation is not to save the realm but to claim the Nymphs Fishing Rod as a reward, which he needs to attract more monsters to his aquarium. Despite his desire to remain anonymous and simply enjoy his hobby, his exploits inadvertently build the reputation of a mysterious figure known as the Underground Knight. The series balances large-scale fantasy battles with slice-of-life ranch management, driven by the protagonists single-minded obsession with monster collecting.