The story begins with Seiga, a young man, fleeing through a forest from a girl named Fiia, who is attacking him with magical projectiles because he ate her cookies. This chaotic chase prompts Seiga to recount how he ended up in this situation.
Seiga was a normal student who died in a traffic accident after a black cat crossed his path. He awoke in a white space where he met Kiyoka Amakawa, a goddess who confessed to accidentally causing his death by spilling tea on his paperwork and then having it discarded as trash. As compensation, Kiyoka offered him reincarnation into a fantasy world, allowing him to choose the Demon Lord's side. After confirming his basic attire (school uniform and sneakers) and lack of possessions, Kiyoka granted him some abilities and sent him off.
Seiga awoke mid-air, falling towards the ground. He found a book titled "Instruction Manual" in his pocket, which contained information on how to fly, though his attempt to use it was comically ineffective, leading to a rough landing. He survived, albeit with a significantly lowered opinion of Kiyoka.
He then encountered Fiia Manius, who revealed herself to be the Demon Lord of Rentarusion, living in the Alma Forest. Fiia explained that her castle was destroyed by a hero a month prior, and she had fled with her subordinates, only to become separated from them. She had been surviving by hunting monsters, aimlessly wandering, until Seiga's dramatic arrival. Seiga, having no knowledge of the world, asked to accompany her, and Fiia, despite her initial hesitation, agreed.
Their journey began, and they soon encountered a slime. Seiga, to Fiia's astonishment, had no combat experience and didn't know about magic. Fiia began teaching him, starting with how to sense mana. Seiga discovered he possessed an unusually large amount of mana, manifesting as a massive, colorless sphere. Fiia explained the color system of magic: red for fire, blue for water, green for wind, and yellow for lightning, with black and white indicating dark and light attributes respectively. She also mentioned that rare individuals could wield two attributes, but light and dark were exclusive. Seiga's colorless sphere was an anomaly, leaving Fiia unsure of his capabilities.
Seiga then experimented with magic, creating a Japanese sword-like weapon from ice, a feat that impressed Fiia as it was considered advanced water magic. He also discovered he could use other elemental magic, like lightning, with surprising power. He learned that magic was cast by imagining the desired outcome and infusing it with mana, often with accompanying incantations.
Later, while searching for a village, they were attacked by a pack of Fog Wolves. Seiga, to Fiia's shock, could communicate with one of the wolves and heal its wounds with his light magic, which was typically used for healing and enchantments. This further solidified Seiga's unique abilities. Fiia then gave the wolf a special collar, a "subjugation item," which allowed it to enter Seiga's shadow, become invisible to mana detection, and not require food. The collar also transformed into a black bracelet on Seiga's arm, signifying his ownership.
Their journey continued, marked by getting lost for three days, depleting their food supplies. They learned that slimes were edible, though tasteless. Seiga, using his newly acquired abilities, captured thirteen slimes by freezing them and then used his fire magic to cook them, adding sugar to make them palatable.
Fiia then explained the properties of various magic attributes and how the wolf's collar would adapt to Seiga's dominant magic. She detailed fire magic as offensive and difficult to control, water magic as versatile with blood manipulation capabilities, wind magic as difficult to master for flight, and lightning magic as unreliable for long-range attacks. She also elaborated on the rare light and dark attributes, with light focusing on healing and enchantments, and dark being incredibly powerful for offense, space manipulation, and assassination. Fiia revealed that demons were originally humans who gained immense magical power, creating the demon realm after being persecuted. She explained that while humans could theoretically use dark magic, their low mana capacity made it impractical, and anyone who could use it was essentially a demon. Beastmen, she added, were mutations of magical beasts.
As the day ended, Fiia, exhausted, went to sleep, and Seiga, with the wolf resting its head on his stomach, also prepared to sleep.