|This article is about a/an episode in Engine Sentai Go-Onger.|
While Renn repairs the Engine Casts, the Ancient Engines give their contemporaries the history of their feud with Horonderthal; they had followed the Ancient Machine Caveman to the Human World during the age of the dinosaurs and had buried Horonderthal, along with themselves, to spare the dinosaurs from further suffering. When Horonderthal resumes his rampage, causing mass chaos in the city with Horonden Waves, Go-on Red intercepts him in Kyoretsu-Oh. However, Horonderthal's rampage was a trap planned by Yogostein for something more sinister; he upgrades Horonderthal with a special device that enables the Machine Caveman to take control of Kyoretsu-Oh, forcing Go-on Red from the cockpit. As Go-on Red battles Yogostein one-on-one, GunBir-Oh and Seiku-Oh battle Horonderthal and the berserk Kyoretsu-Oh. Go-on Blue and Go-on Yellow arrive with their Engine partners and Speedor's Engine Cast. They and Go-on Red assemble Engine-Oh, who engages Kyoretsu-Oh while Horonderthal defeats GunBir-Oh and Seiku-Oh, breaking the two combinations apart. Engine-Oh and Kyoretsu-Oh both connect with devastating punches that break the two combinations and frees Kyoretsu-Oh from Horonderthal's control. The Ancient Engines suggest to the modern Engines that all twelve of them come together in the G12 Formation. The resulting Engine-Oh G12 towers over Horonderthal and scraps him easily. The Go-on teams' victory is short-lived, however, as the Gaiark device Yogostein had managed to implant in Go-on Red during their earlier duel activates; as his teammates watch in horror and disbelief, Sōsuke collapses and is entombed in rust. Yogostein picks up the upgrade that he had crafted for Horonderthal and uses it on himself, vowing to destroy the Human World.
- As part of Super Hero Time, this episode aired alongside Kamen Rider Kiva episode 37, Triangle: Behead the King.
- Engine Eyecatch Race Winner: Engine Gunpherd ("I'm on top!")