"Tekkonkinkreet" is set in the sprawling, chaotic city of Takaramachi, a place teeming with crime, corruption, and urban decay. The story centers on two orphaned street kids, Black and White, who navigate the city's underbelly with a mix of cunning, resilience, and an unbreakable bond. Black, the older and more aggressive of the two, is fiercely protective of White, who is childlike, innocent, and prone to flights of fancy. Together, they rule the streets as the "Cats," a duo feared and respected by both criminals and law enforcement.

The narrative unfolds as a powerful yakuza syndicate, led by the ruthless Snake, begins to tighten its grip on Takaramachi, aiming to transform the city into a sprawling commercial empire. This encroachment threatens the fragile balance of power and the freedom Black and White have carved out for themselves. As the syndicate's influence grows, Black becomes increasingly volatile, driven by a desire to protect White and their way of life. Meanwhile, White's mental state begins to unravel, his innocence clashing with the harsh realities of their world.

Amidst the escalating conflict, a mysterious assassin named Kimura enters the fray, tasked with eliminating the Cats. However, his encounters with Black and White challenge his perceptions and motivations. The story also introduces a weary police officer, Suzuki, who struggles to maintain order in a city spiraling out of control. As the tension mounts, Black and White are forced to confront their own identities and the limits of their bond, culminating in a climactic showdown that tests their resilience and the essence of their relationship.

Themes of duality, survival, and the loss of innocence are woven throughout the narrative, with the city itself serving as a character—a labyrinthine, decaying backdrop that reflects the struggles of its inhabitants. The interplay between Black's hardened pragmatism and White's dreamlike vulnerability drives the emotional core of the story, creating a poignant exploration of brotherhood and the cost of defiance in a world dominated by greed and violence.