Takuto Hasegawa is an ordinary high school student with unremarkable grades, but he harbors a significant secret. He is also a student at a hidden magic academy that exists in a separate dimension, a place not marked on any map where young magicians are trained in the arcane arts. During a retake of a summoning spell exam, his control slips, and the ritual goes disastrously wrong. Instead of a typical familiar, he creates a girl from nothing. This girl, whom he names Tanarotte, is an undifferentiated god-demon with a childlike personality, immense physical strength, and magical power estimated to be over one hundred thousand, enough to destroy an entire country single-handedly. Fortunately for everyone, she immediately declares undying loyalty to Takuto as her creator and master, affectionately treating their bond as that of a newlywed couple.

The story is set across three interconnected worlds: the Human World where Takuto lives a normal life, Heaven the realm of gods and angels, and the Demon Realm. A unique and comedic twist is that both gods and demons have become obsessive otaku, deeply fascinated by human culture, entertainment, and goods, often going to great lengths to smuggle them into their own realms. The Magician's Academy itself is a towering facility in another space, acting as a neutral ground where beings from all three worlds can study. Takuto's life becomes chaotic as he must balance his mundane school life with his magical training, all while managing the powerful and affectionate Tanarotte, who constantly clings to him.

The core conflict arises from Tanarotte's very nature. As an unaligned being with the potential to evolve into either a supreme god or a demon lord, her existence threatens the delicate balance of power between Heaven and the Demon Realm. Both factions become deeply concerned, and representatives from each side, including the angel Miyabi and the demon Synclavia, are dispatched to the human world. Initially sent as assassins to eliminate this potential threat, they ultimately fail and end up living under the same roof as Takuto, working off their failures by helping the eccentric teacher Eitarou Sakuma create doujinshi for comic conventions.

Living in Takuto's already crowded home is his mute and extremely shy cousin, Suzuho Hasegawa, who communicates by writing on a sketchpad. When she unbraids her hair, she transforms into a completely different, rude, and powerful personality known as the Midnight Blue Vampire, who can nullify the magic of anyone around her. Also joining the household is Falcé the Variable Wand, a sentient, god-level magical implement who takes the form of a tall, well-endowed, and ditzy woman. She was a forgotten masterpiece of an ancient alchemist until Takuto stumbled upon her chamber and she accepted him as her new master. This unconventional family is further complicated by Takuto's normal school classmate, Futaba Kirishima, who has a secret crush on him, completely unaware of the magical chaos that defines his home life.

Throughout the narrative, several major arcs unfold. The initial volumes focus on the fallout from Tanarotte's summoning, as Takuto tries to control her power while evading or befriending the agents sent from Heaven and the Demon Realm. A significant middle arc explores the secret of the Original God-Demon Creator, a legendary power that allows a person to create gods and demons. It is revealed that Takuto possesses this dormant ability, which is the true reason Tanarotte was created. This puts him in grave danger, as powerful factions seek to either control or eliminate him before his powers fully awaken. A later arc involves the introduction of other beings with the same power, leading to a confrontation over the fate of the magical world. Amidst the serious battles and political intrigue, the light novels maintain a strong focus on romantic comedy and slapstick humor, driven by Tanarotte's childlike antics, Suzuho's jealous outbursts, and the absurd situations caused by the otaku obsessions of both heavenly and demonic beings.