The narrative opens with Teppei Takamiya, who journeys to China following years of a quiet life on the fictional Kamui Island. He seeks to reunite with his brother Koutarou, who departed for Germany five years prior to study robotics at a university. Tragically, just as his brother commences an exhibition featuring an advanced machine, he is abducted by a wasp-shaped mecha serving the Machine Empire. Teppei initiates a pursuit, utilizing a gauntlet called the "Messiah Fist" to cling to the wasp as it heads toward a clandestine base in the Gobi Desert, which functions as the Empire's headquarters. After being discarded into a garbage heap and confronting a cyborg he somehow recognizes, Teppei's blood activates a relic from the past: a kirin-type B't known only as "X," which was regarded as the most powerful B't in existence five years ago. Both are equally stubborn, though they differ significantly in many respects, and following some initial squabbles, X decides to assist Teppei in his quest to rescue his brother.