"BUILD-DIVIDE -#000000- CODE BLACK" is set in a dystopian city called New Kyoto, where a mysterious card game called "Build Divide" determines the social hierarchy and power dynamics. The city is ruled by the King, an enigmatic figure who enforces his authority through the game. Players, known as "Builders," compete in high-stakes battles using cards that summon creatures and abilities, with the ultimate goal of challenging the King and reshaping the world.

The protagonist, Teruto Kurabe, is a young man with no memory of his past. He awakens in New Kyoto and is quickly drawn into the world of Build Divide after encountering a skilled Builder named Sakura Banka. Sakura explains that the game is the key to uncovering the truth about the city and its oppressive ruler. Teruto, driven by a desire to regain his memories and understand his purpose, decides to compete in Build Divide, despite his lack of experience.

As Teruto progresses through the game, he forms alliances with other Builders, each with their own motivations and backstories. These include Rei Hizumi, a calm and strategic player, and Kikka Hizumi, Rei’s younger sister, who is fiercely protective of her brother. Together, they navigate the dangerous and competitive landscape of Build Divide, facing off against powerful opponents and uncovering secrets about the game’s origins and the King’s true intentions.

The narrative explores themes of power, identity, and rebellion, as Teruto and his allies challenge the established order and confront the King’s enforcers, known as the "Royal Guards." Each victory brings them closer to the truth, but also reveals the darker aspects of the game and the sacrifices required to change the system. The story builds toward a climactic confrontation with the King, where the fate of New Kyoto and its inhabitants hangs in the balance.