A kid shows up to see his brother after eleven years and walks into a kidnapping. That single moment of timing, arriving just late enough to matter, breaks everything open. B't X turns on that specific kind of loss: Teppei doesn't get the reunion he trained his whole life for, he gets a mission instead, and a broken machine that only works because his blood runs through it decides to follow him anyway. The Machine Empire isn't some distant threat. It's the reason his brother is gone, and every fight to get him back costs something. This is a show about a kid who wanted to come home and ended up with a robot partner and no other choice.
Teppei has spent eleven years in isolation, training under a mysterious mentor named Karen. When he finally gets the chance to reunite with his older brother Kotaro at a scientific conference, everything falls apart in seconds. A woman named Aramis orchestrates a massacre and abducts Kotaro, vanishing with him into the shadows. Desperate to get his brother back, Teppei pursues them across dangerous terrain and stumbles upon B't X, a damaged mechanical horse left for dead. Something extraordinary happens when Teppei's blood touches the machine: B't X awakens, recognizing him as its destined partner. With his new mechanical companion by his side, Teppei finds himself drawn into a larger conflict involving a shadowy organization called the Machine Empire. As he fights to track down Kotaro, Teppei uncovers secrets about his past, his training, and why his brother was targeted in the first place. The bond between boy and beast becomes the key to surviving encounters with increasingly dangerous enemies, each encounter revealing deeper layers to the conspiracy holding his brother captive.