Katsuki, a hopeless man with a dream of getting intimate with monster girls, suddenly finds a portal to another world appearing in his home! However, the other side of the portal leads to the deepest level of a brutal dungeon, where powerful monster girls dominate in a survival-of-the-fittest battle. Granted an absolute stealth skill that makes him completely unperceivable, Katsuki, despite being only Level 1, boldly steps into the dangerous dungeon for the sole purpose of getting close to monster girls! Using his ability, he teases, tames, and claims monster girls without them noticing—thus the most degenerate tamer is born!