The protagonist, a legendary assassin known for his efficiency and discretion, decides to retire after a successful final mission. He plans to become a teacher at the academy where he was trained, nurturing young talent. However, his retirement is violently interrupted when the passenger plane he is on is attacked by fighter jets. Realizing he is being eliminated, he attempts to save himself but the plane is shot down.
He awakens in a white, temple-like structure and meets a beautiful, all-white goddess. She explains that he died and offers him a choice: reincarnation as someone new or a second life in another world in exchange for completing a task. The task is to assassinate a hero in a sword-and-magic fantasy world eighteen years after his reincarnation. The hero, after saving the world from a demon lord, will become a tyrannical ruler and lead to its destruction. The goddess chooses him because only an assassin, operating within human limits, can kill such an overpowered individual. She grants him the ability to choose five skills for his reincarnation. He chooses "Super Regeneration" (S-rank), "Pattern Weaver" (A-rank, allowing him to create magic), "Growth Limit Break" (B-rank), "Body Arts" (C-rank, for its versatility), and an interesting D-rank skill. He is reincarnated into the Touhaade family, an assassination noble house in the Alvan Kingdom, as the heir, Lug.
Five years pass, and Lug, with his exceptional skills and the benefits of "Super Regeneration," excels in his training. He learns that the Touhaade family secretly eliminates threats to the kingdom, balancing their role as healers. He also discovers his latent magical abilities and, with the help of his father, receives the Touhaade family's unique "Magic Eye," which enhances his vision and allows him to see magic. He also begins learning magic from a prodigy named Dia Vicone, who is ten years old but possesses immense magical talent and a high-ranking noble background. Lug's chosen skills and his innate abilities allow him to quickly master magic, even developing new spells by modifying existing ones. He creates powerful weapons, including a firearm-like magical device called "Gunfire" and a much larger "Artillery," capable of immense destruction. He also develops the ability to create alloys, crafting superior weapons like titanium and tungsten blades.
Lug's parents, while loving and supportive, are also formidable figures in the Touhaade family. His mother, Esri, is a skilled healer and a surprisingly adept cook, while his father, Kian, is a master assassin and the head of the family. They guide Lug's development, emphasizing the importance of both combat prowess and a strong moral compass, even when performing assassinations. Lug also develops a close bond with Dia, who becomes his friend and collaborator in magic research. He learns about her noble family, the Vicone, and suspects a connection to his own mother.
As Lug approaches his tenth birthday, he continues to hone his skills and develop new magic, preparing for the hero's eventual rise. He has also acquired a loyal subordinate, Tart, a former orphan with magical potential whom he found in the forest and trained as his personal assistant. Lug's ultimate goal remains the assassination of the hero, but he also seeks to live a fulfilling life, learn about love and happiness, and build a team to support him in his mission. The narrative sets the stage for Lug's future endeavors, hinting at the challenges and growth he will experience as he navigates his destiny.