A young knight, faithfully fulfilling his duty to protect humanity, meets a tragic end while battling demons. In an ironic twist of fate, he is reincarnated in the lowest levels of a dungeon as a Behemoth—an S-class monster of immense power and one of the very creatures he once swore to defend humans against.
Starting out as a juvenile, he looks deceptively like an ordinary orange cat. Despite his seemingly overwhelming ability to absorb the skills of any creature he devours, his escape from the dungeon leaves him gravely injured. Just then, Aria, a D-rank adventurer, discovers him, decides to nurse him back to health, and eventually adopts him as her pet, giving him the name Tama.
Recognizing Aria as his master and bound by his knightly honor from his past life, Tama vows to protect her from any danger, even if it means risking this second chance at life.