考古学者の司馬遷次郎は発掘した銅鐸から、古代日本を支配した邪悪な国家「邪魔大王国（じゃまたいおうこく」と「女王ヒミカ」の復活を察知していたが、ある日突然、銅鐸の奪還をもくろむヒミカ配下の者たちの襲撃を受け死亡する。だが優秀な科学者でもあった遷次郎は、予めコンピュータ・マシンファーザーに自身の意識と記憶を残し、邪魔大王国に対抗するための基地ビルド・ベースと巨大ロボット・鋼鉄ジーグを建造して、来るべき日に備えていた。一方、カーレースで大事故を起こしながら無傷であった息子の宙は、既に自身がサイボーグへと改造されていたこと、「鋼鉄ジーグ」として邪魔大王国と戦う宿命にあることをマシンファーザーに告げられる。宙が頭部に変身し、磁石の力で全身のパーツと合体することで完成する「鋼鉄ジーグ」は、日本の支配をもくろむ邪魔大王国と、その巨大兵器・ハニワ幻人（げんじん）に立ち向かう。
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Archaeologist Shiba Senjirō had detected the revival of the evil nation “Jamataiōkoku” and its “Queen Himika,” who once ruled ancient Japan, through excavated bronze bells. One day, he was suddenly attacked and killed by Himika's subordinates plotting to seize the bells. However, Senjiro, who was also an outstanding scientist, had preemptively stored his consciousness and memories within the computer Machine Father. He had constructed the base Build Base and the giant robot Steel Zieg to oppose the Jamata Kingdom, preparing for the day that would come. Meanwhile, his son Sora, who had survived a major car accident unscathed, was informed by Machine Father that he had already been transformed into a cyborg and was destined to fight the Joma Empire as “Steel Zieg”. Zuu transforms into the head section, and Steel Zieg is completed when he combines with the body parts using magnetic force. He confronts the Great Kingdom of Jyama, which plots to dominate Japan, and its giant weapon, the Haniwa Phantom.
