Shinobu Iijima and his younger sister Megumi are taken in by their father’s old friend, Ayumu Imabayashi, after a tragic accident claims their parent. Shinobu works tirelessly to build a new life, finding solace only in his sister and the mobile game La Vita Online. His world shatters when Ayumu reveals his true nature, admitting he intends to break Shinobu and claim Megumi for himself. Before Shinobu can process this betrayal, Ayumu deletes all his game data. Moments later, their high-rise apartment is consumed by fire, and Shinobu falls to his death while trying to protect his sister.

He awakens in a fantastical realm and quickly realizes he has been reincarnated into the world of La Vita Online. A divine announcement confirms his worst fears: his foster father Ayumu is already here, having also been reincarnated. A cruel countdown begins to tick, marking the time until Megumi, still clinging to life in the real world, will succumb to her injuries and join them. Stripped of all his previous progress and reset to level one, Shinobu faces a desperate race. He vows to protect his sister at any cost, even if it means crawling up from the bottom to battle not only his treacherous foster father but also the other ruthless reincarnated players who now populate this world.

Shinobu discovers he possesses a unique advantage that sets him apart from every other reincarnator: access to a hidden dungeon known only to him. Within this secret space, he grinds relentlessly, harnessing the power of a summoner to acquire legendary allies and build strength at an unprecedented rate. He forms a growing community of followers at his home base, which he names Izanach. Early in his journey, he rescues a young girl named Alice from a slave market. Believing herself useless after failing to contract with the Four Great Spirits, Alice is shown a shocking truth by Shinobu and joins his ranks, eventually returning to her magic academy to hone her skills against former rivals.

The hunt for his foster father, Imabayashi, becomes a central quest. Shinobu learns of a mysterious young girl, a genius mage who has created her own rune system that appears to reincarnated players as Japanese characters. This girl bears a striking resemblance to his sister Megumi. Vowing to follow any lead, Shinobu travels with his companions to Abrasil, the home of Imabayashi’s powerful guild, the Lions of Patriotism. There, he must uncover the guild’s illegal activities in a race against time to find the mysterious girl before his evil foster father can get to her.

After the eventual death of his archnemesis, Shinobu’s long-lost sister Megumi vanishes from his arms once more, leaving him desolate. As he accepts the love and support of his comrade Kaori, other members of his group, including the young apprentice Alice, struggle with their own feelings. A world-shaking announcement then declares that La Vita Online is entering its final phase. The Agartha event is on the horizon, giving the players a final deadline to tackle the ultimate dungeon and beat the game. Shinobu and his comrades must tie up loose ends and deal with rogue elements before taking the final leap into Agartha, where the fate of everyone he loves will be decided.