A man awakens to find himself reincarnated not as the hero of his favorite video game series, Spirit Wars: Dungeon Magia, but as its most pathetic character. He is Kyouichirou, a bottom-feeder tutorial boss so weak his signature catchphrase is a meek Nyahaha, and his fate is to be easily slaughtered by new players within the first few minutes of the game.

However, his despair turns into fierce determination when he discovers that in this world, his beloved older sister, Haruka Aono, is fated to die from an incurable illness. Unable to accept this grim outcome, he makes a solemn vow to tear apart the original scenario piece by piece. Armed with encyclopedic knowledge of the game's every secret, hidden route, and tragic plot point, this self-proclaimed weakest boss decides the only way to save his sister is to recruit an unlikely army: the very bosses he once admired from the screen.

His mission is to find and befriend the games most dangerous characters before the hero can cut them down. This includes seeking out the strongest secret boss and the heart-wrenching boss from the saddest route, all in an effort to change their predetermined, bloody fates. Successfully rescuing Haruka from her scheduled death is his first major victory, and together they form an adventuring party, combining his intimate game knowledge with her unparalleled swordsmanship to tackle other unfair challenges the world throws at them.

Their efforts soon attract the attention of powerful figures, such as James Szilard, the head of one of the five great clans and one of the strongest characters in the entire game, who challenges them directly. As Kyouichirou's party grows, he recruits Jupiter, a girl destined to become the terrifying boss of the game's true route. The party learns her villainous behavior is not her fault but the result of a monstrous spirit, Keraunos, the beast of black lightning, sleeping inside her. They set out to perform a dangerous cleansing ritual to save her, but are interrupted by a figure from Jupiter's past, revealing deeper secrets about her contract with the beast and the true nature of the world they are trying to change.