Teppei is going to visit his older brother Kotaro Takamiya during a scientific conference after training with a mysterious woman named Karen for the past 11 years. However his plans for a family reunion become stalled when a woman named Aramis kills everyone in the conference to kidnap Kotaro. Teppei on the way to save his brother encounters a broken mechanical horse called B't X who revives thanks to Teppei's blood and is now his aide to save his brother from the mysterious Machine Empire.

Teppei have managed to defeat Machine Empire’s forces time and again, escalating his rescue attempt of his brother into an all out war! His courage has even earned the support of the Empire’s Spiritual Guardians. Together with Teppei, they now seek to learn Machine Empire’s true motives and put an end to their rampant destruction.
 
(Source: TMS)

Kotaro is a robotics and technology genius who has become the object of an evil group's attention. They want Kotaro to help them build their Machine Empire and beef up their artificially intelligent creations, the B'ts (pronounced "beats"). Once the Empire captures Kotaro, he may have no choice but to work for them, unless his kid brother Teppei can somehow rescue him. But after a powerful B't and its master attack Teppei, it appears Kotaro's last hope is lost. However, Teppei's spilled blood awakens a legendary B't (these evil creatures bond with blood), so Teppei now has one of the most deadly, sentient creatures-the B't X-at his command. Still, the brothers are up against an army from the Machine Empire, and the odds are against them. Even if they survive, B't X isn't keen on going up against another creature from the Machine Empire. (Source: Tokyopop)

B'T Shadow X