Reborn as a dungeon master in another world, Akira Hanamura quickly constructs a comfortable dungeon within his first 30 minutes in this new realm. However, he soon realizes his creation is a "zero-defense" setup—vulnerable to the constant influx of adventurers and, if conquered, spells instant death for him. Faced with this dire predicament, Akira must use his unique Reproduction skill to multiply his monster minions and fortify the dungeon. With each encounter, he gradually transforms his humble lair into an ultimate stronghold, utilizing strategies, monster alliances, and swift adaptability to fend off every challenge.