Several hundred years in the future, the resources of the Earth runs out, and the progression of the human race has stagnated. 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," in which 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 feelings of an organic life. The size and shape of the resulting product seem to be different 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. (Source: ANN)

Van, a lanky and apathetic swordsman, is on a journey to kill the murderer of his fiancé. The only characteristic he has to go by is that the murderer has a claw for an arm, hence the murderer being referred to as The Claw Man. During his travels, Van happens to pass through the city of Evergreen, which is defending itself from bandits who aim to rob the city of its treasury. It is in this city that Van meets Wendy Garret, a timid young girl who is looking for her kidnapped brother. When the city pleads for Van's assistance to defend it, he refuses, claiming it has nothing to do with him and thus leaves the city on its own to deal with the peril. Soon after, Van comes across the raiding bandits himself and they eventually tick off the swordsman to a degree where he takes action against them for his own personal vendetta. Surprisingly, Van learns that the bandits had ties with The Claw Man, and in kidnapping Wendy's brother for a reason they did not disclose. After the bandits are dealt with easily, Van and, much to his chagrin, Wendy continue the journey in search of The Claw Man. Little do they know, however, that The Claw Man is involved with something more atrocious than either could fathom. [Written by MAL Rewrite]

Technology, science, and industry—this is 1969 Tokyo, and there is no better time to be alive! But a shadow looms quietly around every corner: "Invaders" have infiltrated the populace, and nobody knows who or what they are. Only the members of the top-secret agency AEGIS know of their existence. Covertly fighting the enemy is their job, but only those with the ability to open "Gates" to another world can truly defeat them. Within AEGIS is a specialized task force known as the "Gate Keepers." Composed of extraordinary individuals with a variety of Gate-related abilities, they are the only ones who can save humanity from the vicious Invaders plaguing the planet. Shun Ukiya is an average high school student who lives with his widowed mother and little sister. While on his way home from school one day, he comes across a group of Invaders heading toward his house. In a desperate plight to save his family, Shun discovers he possesses the ability to open a Gate, allowing him to harness massive amounts of energy. With his newfound ability exposed, he catches the attention of AEGIS, and particularly the interest of one of its Gate Keepers, Shun's childhood friend Ruriko Ikusawa. [Written by MAL Rewrite]

Thirty-two years have passed since the initial defeat of the Invaders. A.E.G.I.S is all but dismantled now and has become an underground organization consisting of only a few Gate Keepers left to save humanity. One of the members, Ayane Isuzu, is sick of society as a whole, though she continues to defeat the Invaders out of spite for her father. Suddenly, she learns of the revival of Machine General (Kikai Shogun) and Devil Count (Akuma Hakushaku), but a mysterious ghost girl now accompanies them. It is up to Ayane and the remnants of A.E.G.I.S to stop them and save humanity once again. (Source: ANN)