Hiroshi Shiba is an outstanding car racer whose father is assassinated upon the discovery of the Bronze Bell from an ancient civilization. This Bronze Bell is coveted by Queen Himika who comes from that ancient era; the Yamatai Kingdom, and whose ambitions are to seize the Bell and dominate earth.

Hiroshi is left with a few items from his father which would enable him to morph into a full sized mecha warrior, the Steel Jeeg. With these abilities and the help of his father's brain implanted in a computer and his assistance Miwa "Micchi" Uzuki, they set out to neutralize Himika's plans of world dominion.

Kotetsushin Jeeg takes place fifty years after the original and features a new cast of characters—primarily the new main character Kenji Kusanagi, a high school student who becomes Kotetsushin Jeeg to fight the sudden reappearance of "Haniwa Genjin" ("Haniwa Phantom Gods", or clay robots) from the Jamada Empire ruled by Queen Himika. Other characters include Tsubaki Tamashiro (grandaughter of Miwa Uzuki); and Kyou Misumi. Other main characters from the original series also appear.