Gun-ota ga Mahou Sekai ni Tensei Shitara, Gendai Heiki de Guntai Harem o Tsukucchaimashita!?

Horita Hata, a man killed by his former bully with a kitchen knife, awakens as a baby in a strange world. He lies beside a silver-haired, dog-eared infant in a fluffy bed and realizes he has been reincarnated. A woman with rabbit ears and pink hair, later known as El-sensei, picks him up gently. He sees his reflection as a black-haired baby and accepts his new life.

Raised in an orphanage in the small town of Hoard, he learns the local language over the next year. He deduces that he was abandoned shortly after birth with a handkerchief bearing the name "Lute." He resolves to live differently in this world: to become strong, never run from hardship, and always help those in need as atonement for his past cowardice, which allowed a classmate to be driven to suicide and led to his own murder.

At age three, Lute can walk and talk. He attends El-sensei's classes on reading, writing, arithmetic, history, and general knowledge. He learns that this world was created by a god called Tenjin-sama, but six Demon Lords stole divine law and turned it into magic. Heroes of five races—Human, Fairy, Beastkin, Demon, and Dragonkin—stole magic back and sealed five Demon Lords; the last king still lurks on the Monster Continent. Magic is divided into elemental, non-elemental, healing, and support types, and magicians are ranked from C- up to SSS. Those of B- talent or higher are considered elite.

Lute asks El-sensei about his own magic talent. She hesitantly tells him he has no talent as a magician and his capacity is around C Rank, the second lowest. She advises him to accept reality and choose another path. Lute initially feels disappointment but then hears about magic tools in class, especially Magic Liquid Metal—a substance that can form any weapon when magic power is channeled into it. He envisions creating a handgun, a modern weapon that could give him an edge. To do this, he must learn to control magic power even if he cannot become a full magician.

He begins attending basic magic classes on the sidelines. After observing lessons on Resistance Formation and Body Reinforcement Technique, he attempts to use them himself. During his first try, he successfully creates a shining rhombus of resistance magic, but he loses all magical power in an instant, faints, and soils himself. El-sensei rushes to help. Later, he tries Body Reinforcement, successfully enhancing his jump, but again exhausts his magic power, falls backward, hits his head on a stone, bleeds profusely, loses bladder control, and chokes on vomit. El-sensei intervenes to save his life and then scolds him angrily. She bans him from further participation in magic classes due to the dangers and disruptions.

Despite these setbacks, Lute does not give up his goal. He still believes that mastering magic power is essential for using Magic Liquid Metal to create a handgun. His determination remains strong even as he is forced to step back from direct training. He knows he must find another way to become powerful enough to protect others and atone for his past sins.