In a future far beyond our own, the Earth has been drained of its resources and humanity has been trapped in a cycle of stagnation. In order to survive, the world has been fragmented into "Units." Hajiki Sanada lives in Unit 74, located in a place called "Night Town," with his mother and sister. At the stroke of midnight, Night Town is plunged into darkness as all of its electricity is shut down. However, the key to this story lies in an extraordinary object called GAD, which has the power to reconstruct matter based on the emotions of organic lifeforms. The size and shape of the resulting product are determined by the feelings of the lifeform. When Hajiki inadvertently encounters a GAD, it transforms into a colossal robot - a Tekkoudo or "Iron Giant" - that he names Lightning. As he discovers others who possess Tekkoudos similar to his own, he realizes that he must come to terms with these "kin" in order to find a way forward.