Our protagonist, a 34-year-old NEET, finds himself utterly broken and homeless after his family violently casts him out. He's a man defined by regret, haunted by a past humiliation that drove him into a two-decade-long reclusive existence. Yet, in his final moments, a scene unfolds that demands a choice: three high schoolers are about to be struck by a speeding truck. Driven by a desperate urge to die with at least one good deed, he lunges, saving two, only to be crushed himself. This act of sacrifice, a stark contrast to his self-absorbed life, is immediately rewarded with an impossible second chance. He awakens as a baby, Rudeus Greyrat, in a world where language is foreign, and his new parents, Paul and Zenith, are adventurers. The world itself is a clean slate, free from the social baggage of his past. This profound rebirth, retaining his adult intellect, fuels a fierce resolve: he will not squander this life. He will live earnestly, no longer a useless shut-in, but a man who strives for something meaningful. His immediate challenge is to adapt, to understand this new world, and to master its rules, starting with basic communication.

Rudeus, now a baby, immerses himself in learning his new world's language by devouring every book in his rural home. His innate talent for observation, combined with his adult intellect, leads to a groundbreaking discovery: magic can be cast without incantations, a technique he dubs "voiceless incantation." This personal cheat, which allows him to cast spells faster and more efficiently than others, propels his magical growth exponentially. His accidental destruction of a wall with an intermediate water spell reveals his prodigious talent to his parents. Far from being angry, his mother Zenith is overjoyed, immediately pushing for a magic tutor. His father Paul, a skilled swordsman, insists he also train in the blade. This familial dynamic brings two pivotal mentors into Rudeus's life: Roxy Migurdia, a young but powerful Water-Saint ranked magician, and Paul himself for swordsmanship. Roxy, initially skeptical, is stunned by Rudeus's voiceless incantation and becomes a dedicated, if somewhat overwhelmed, teacher. This period marks a foundational acceleration in Rudeus's power and skill, but also forces him to confront his lingering social anxieties as he interacts with his first teachers and learns the world's social nuances.

As Rudeus grows, his training intensifies, but so do his emotional challenges. He forms a close bond with Sylphiette, a green-haired half-elf girl he initially mistakes for a boy due to her appearance and the village's prejudice. He protects her from bullies and teaches her magic, fulfilling his vow to live earnestly. A significant turning point arises when his father, Paul, is caught in an affair with the maid, Lilia. Rudeus, using his adult cunning, skillfully manipulates the family discussion, deflecting blame from Lilia and ensuring she and her unborn child remain with the family, subtly showcasing his moral compass and persuasive abilities. This moment of family crisis highlights Rudeus's growth beyond his NEET past, as he actively shapes his new family's future. Later, as Rudeus grows closer to Sylphy, Paul orchestrates a forced separation, sending Rudeus to the city of Roa as a tutor for his grand-niece, Eris Boreas Greyrat. This drastic measure is meant to prevent both Rudeus and Sylphy from becoming overly dependent. The encounter with Eris is explosive: a violent, arrogant noble girl who initially beats Rudeus. His cunning comes to the fore as he devises a "kidnapping" plan to humble her and earn her respect, leading to a profound shift in their relationship and Eris's willingness to learn.

Rudeus's life as Eris's tutor in Roa progresses, with Eris slowly learning and even showing affection. On Rudeus's tenth birthday, a grand party is held, culminating in a precious gift: a custom-made magic wand. However, the joyous occasion is shattered by an unprecedented global phenomenon: the Mana Calamity. A blinding white light engulfs the city, teleporting Rudeus and Eris to the desolate Magic Continent, far from home and any familiar faces. This catastrophic event tests Rudeus's resolve immediately. He awakens beside a campfire, only to find Eris, and a terrifying figure: Ruijerd Superdia, a legendary warrior of the feared Superd race, marked by emerald hair and a ruby-like third eye. Despite Roxy's warnings about the Superd's brutality, Rudeus, guided by a cryptic dream message from the enigmatic Human God, chooses to trust Ruijerd. He learns of the Superd's tragic history, framed as villains by Laplace, and Ruijerd's lifelong quest to clear his race's name. In a pivotal decision, Rudeus commits himself and Eris to helping Ruijerd, forming a new adventuring party named "Dead End." This alliance marks the beginning of their arduous journey back home, interwoven with a mission to redeem a demon race.