The protagonist, formerly a socially awkward Japanese high school student named Nanairo Raku, finds himself reincarnated as Luluc, a five-year-old boy in a fantasy world. Raku, who died absurdly when a helicopter crashed on his graduation day, now inhabits the frail body of Luluc Mutel, a ducal heir.
Luluc quickly learns his new family is dysfunctional. His father, Duke Digray Mutel, a powerful knight, harbors deep resentment towards Luluc, whose birth caused his first wife's death. Luluc's stepmothers also treat him with disdain. His older half-brother, Gaui, a boisterous but dim-witted seven-year-old, frequently bullies Luluc but possesses a fierce sister complex for their four-year-old half-sister, Lilith. Lilith, initially shy and scared, soon develops an affectionate bond with Luluc, contrasting with her dislike for Gaui.
Luluc discovers he was born with "Magic Element Deficiency Syndrome," a fatal condition that prevented him from using magic and would have killed him by age six. However, his reincarnation seems to have purged the "magic element poison" from his body, though he still lacks magic. This is revealed by Vergana, the blind, muscular maid chief and combat instructor, who, despite her blindness, is incredibly perceptive and strict. She immediately puts Luluc and Gaui through rigorous physical training, much to Luluc's dismay.
Exploring the vast Mutel mansion, Luluc uncovers "magic circles" integrated into everyday objects, signifying a world where magic functions as technology. He also accesses a game-like "status" system, revealing his current abilities and a passive skill, 'Calm and Collected.' Luluc overhears his father and the butler plotting to marry him off as an adopted son-in-law to a distant, eccentric viscount family when he turns ten, effectively disowning him. Determined to avoid this fate, Luluc resolves to gain strength and knowledge. He spends his days training with Vergana and devouring books in the mansion's extensive study, where Lilith often secretly observes him, eventually joining him for tea and reading.
With his father away on a year-long tour, Luluc seizes an opportunity to visit a church for a status appraisal, subtly aided by Vergana. He sneaks into a cargo carriage heading to the city, only to find Gaui already hiding there, attempting to secretly follow Lilith and their mother, Leena. In the bustling city, Luluc learns about dungeons and a magic-driven economy. However, their outing takes a dire turn when Lilith goes missing. Luluc, displaying unexpected leadership, calms a distraught Leena and organizes a search with Gaui and the accompanying soldiers.
Following Gaui's intuition, they locate Lilith, bound and gagged by three kidnappers in a dark alley. Gaui bravely charges, but the sight of a kidnapper's knife paralyzes both boys with fear. At this critical moment, Luluc's unique skill, 'Calm and Collected,' activates, clearing his mind. Remembering his promise to Lilith, Luluc snatches Gaui's wooden sword, his fear replaced by a fierce resolve to rescue his sister, marking a turning point from passive observation to active engagement in this dangerous new world.
