Several hundred years into the future, Earth's resources have vanished and human progress has stalled. Society is divided into “Units,” and the story follows a boy named Hajiki Sanada, who lives with his mother and sister in Unit 74’s Night Town, a place where all electricity shuts off at midnight. Central to the plot is the GAD, an object that can reconstruct materials by reacting to the emotions of living beings. The size and shape of what the GAD produces vary according to the feelings it detects. When Hajiki accidentally comes into contact with a GAD, it transforms into a colossal robot, a Tekkoudo, or “Iron Giant” – which he names Lightnin.