Several centuries from now, Earth's resources run out, and the advancement of the human race stagnates. The world is now divided into "Units." A boy named Hajiki Sanada lives with his mother and sister in Unit 74, in a place called "Night Town," where all electricity is shut down at midnight. The key in this story is an object called the GAD. GADs have the ability to reconstruct materials while reacting to the feelings of an organic life. The size and shape of the resulting product seem to differ depending on the kinds of feelings that the life possesses. When Hajiki comes in contact with one by accident, it transforms into a huge robot—a Tekkoudo, or "Iron Giant"—which Hajiki names Lightning. And soon he realizes that he isn't the only one with a Tekkoudo, and must find out how to deal with those others who he feels are the "same" as himself.