Kenichi Yamada has spent most of his adult life as a dedicated online gamer, but the servers for the game he played religiously are shutting down, leaving him with a void to fill. Finding modern games far too easy, he yearns for the punishing, thousand-hour challenges of the past. He stumbles upon a mysterious, untitled game that promises an incomparable challenge with no end. Drawn by an invitation that reads You are invited to a game that will never end, Kenichi selects the Hell Mode difficulty without hesitation and chooses the newly released Summoner class, known for its absurdly high difficulty. Instead of playing a game, he finds himself reincarnated in another world as Allen, an infant born into a family of serfs.

The world Allen now inhabits operates like a game, complete with levels, stats, skills, and job classes, but the brutal reality of Hell Mode becomes apparent immediately. Leveling up is excruciatingly slow, and his Summoner class starts with only the ability to call forth weak creatures like mice and frogs, leaving him far less capable than non-magical peers his age. Without walkthroughs or guides, Allen must experiment and grind relentlessly to understand his abilities and grow stronger. However, his adult gamer mind and strategic thinking give him an edge, allowing him to develop creative hunting methods and grind more efficiently than anyone expects.

The story follows Allen's journey from infancy through childhood as he navigates the harsh social hierarchy of serfdom. Key companions include Krena, a spirited and battle-hungry childhood friend who becomes a loyal ally, and Cecil, a noble's daughter he encounters. Other notable characters include the warrior Dogora, who joins Allen's party. The narrative progresses through distinct arcs, beginning with Allen's early years as he learns to walk, talk, and secretly grind his Summoner skills. He eventually reveals his abilities to his family, leading to an Appraisal Ceremony that exposes the true brutal difficulty of his path.

As Allen grows, his monster hunting skills earn him the attention of the local baron, leading to a promotion and a new life serving the Gravelle noble family. He faces escalating threats, including a devastating attack on the city by a B-rank monster called a Murdergalsh, which forces him to unlock his class's true potential. Later episodes see Allen undertaking missions against threats like an Armored Ant colony, victories that begin to have unforeseen political consequences. Throughout these challenges, Allen discovers that the world's difficulty settings are not merely game mechanics but are woven into the fabric of society, with those in easier modes often leading privileged lives while Hell Mode players face relentless hardship and must rely on sheer perseverance, intelligence, and the bonds they form to overcome the unbalanced system designed against them.