Labyrinth City, Lewin. Many adventurers gather in this town with a labyrinth infested with monsters. The protagonist, who lost his life and was reincarnated, wakes up in the labyrinth. His appearance is that of a kitten!? However, his true identity is the juvenile form of a disaster-level S-rank monster, Behemoth! He meets Aria, a budding D-rank adventurer. Mistaking him for a kitten, Aria takes him home, where he is seriously injured, treats him, names him "Tama" and makes him her pet. Touched by his kindness, Tama vows to become a knight who will protect Aria. Tama uses his S-rank power to adventure in the labyrinth with Aria. He meets unique companions and is sometimes doted on, and his days as a knight (pet) begin!