Labyrinth City—Ryuuin. This town, home to a monster-infested labyrinth, attracts countless adventurers. The protagonist, who died and was reborn, awakens within the labyrinth—but his appearance is unmistakably that of a kitten!? However, his true identity is that of a juvenile form of the calamity-level S-rank monster, "Behemoth"! He encounters a rookie D-rank adventurer named Aria. Mistaking him for a kitten, Aria takes the severely injured creature home, tends to his wounds, names him "Tama," and adopts him as her pet. Touched by her kindness, Tama vows to become Aria’s knight and protector. Utilizing his S-rank powers, Tama adventures through the labyrinth alongside Aria. Meeting a cast of unique companions, he begins his days as a knight (pet)—sometimes even being utterly doted on!