"BASTARD!! -Heavy Metal, Dark Fantasy-" is set in a post-apocalyptic world where magic and technology coexist. The narrative revolves around the Kingdom of Metallicana, which faces destruction at the hands of the evil Anthrasax, four powerful demon kings. To combat this threat, the kingdom's high priest, Geo, resurrects Dark Schneider, a notorious and immensely powerful sorcerer who was sealed away years ago due to his destructive tendencies. Dark Schneider, often referred to as D.S., is a morally ambiguous character with a penchant for chaos, yet his unparalleled magical abilities make him the kingdom's only hope.

The story begins with the resurrection of Dark Schneider, who initially appears as a child due to the incomplete ritual. As he regains his full power, his arrogant and lecherous personality resurfaces, complicating his alliance with the kingdom. Despite his flaws, D.S. becomes a reluctant protector of the kingdom, particularly of Yoko, a young warrior priestess who shares a complicated history with him. Yoko serves as a moral counterbalance to D.S., often challenging his selfishness and pushing him toward more heroic actions.

As the series progresses, Dark Schneider and his allies confront the Anthrasax and their minions, engaging in intense battles that blend high-octane action with dark fantasy elements. The narrative explores themes of power, redemption, and the duality of human nature, as D.S. oscillates between his destructive impulses and moments of genuine heroism. Supporting characters, such as Arshes Nei, a former disciple of D.S. with her own tragic backstory, add depth to the story, highlighting the complexities of loyalty and betrayal.

The plot is driven by a mix of episodic conflicts and overarching storylines, with each encounter revealing more about the world's lore and the characters' motivations. The series balances its dark, often violent tone with moments of humor and absurdity, particularly through D.S.'s over-the-top personality and interactions with other characters. The blend of heavy metal aesthetics, intricate magic systems, and a richly detailed fantasy world creates a unique atmosphere that distinguishes the series within the genre.