A boy who retains the memories of his previous life as a bullied Japanese high school student is reincarnated into a fantasy world. He is born as Nyango, a member of the black cat kin, a beastman race considered one of the weakest due to their lack of physical strength and lazy disposition. Despite this significant handicap, he aspires to become a powerful adventurer. His magical attribute is determined to be Sky, a power so rare it is initially ridiculed as useless, an empty attribute of no value.

The story is set in a world populated by beastmen, beginning in the protagonist's hometown of Atsuuka Village. Refusing to accept the limitations of his race and his perceived weak magic, Nyango dedicates himself to rigorous training in the surrounding mountains. He learns physical enhancement magic and staff combat from the village bodyguard, Zeol. His determination is tested when he loses the sight in his left eye while rescuing the village chief's son, Miguel, from a kobold attack. Unwilling to give up, Nyango develops new, unconventional methods to increase his power, including the forbidden practice of eating raw monster hearts to boost his magical abilities.

As Nyango grows stronger, his efforts begin to pay off. He attracts the attention of a veteran adventurer team and shifts his field of activity to the larger town of Iburo. There, he aims to join the adventurer party called Chariot. When he arrives, he discovers the party is away on a quest. However, a professor at the local Academy, intrigued by Nyango's rare wind magic, offers him a personal request. The reward for completing this task is learning new sigil magic, setting him on a path to becoming one of the strongest adventurers.

Nyango's career progresses at an unprecedented pace. He achieves Iron-rank by defeating a Bronze Wolf, earns Copper-rank for his contribution to rescuing students during the Iburo School Raid, and is promoted to Silver-rank after slaying a Wyvern. His courage and sense of justice lead him to protect Princess Hermeline from an attack by an anti-aristocratic faction. In recognition of his deeds, the King grants Nyango a knighthood and the family name Elmer, making him a noble known as Sir Elmer, the Unfalling Magic Cannon User. During this time, the princess uses her newly awakened light-attribute magic to heal Nyango's damaged left eye. The eye is restored with a new blue color, contrasting with his original gold, and it awakens as a Magic Eye capable of seeing the flow of mana. Later, at the royal academy, a professor asks him to investigate mysterious magical circles. Nyango identifies a magic power recovery circle and learns to form it around his own body, granting him immense magical stamina, though this technique has the fatal flaw of dispelling all active spells if he experiences a sudden shock.

A reboot version of the manga series also exists, retelling the story of the boy reincarnated as the weakest cat person who, through ingenuity, learns to use his dismissed empty attribute to create weapons and armor from sky magic and even walk on air, embarking on his journey to become the strongest catman adventurer.