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 the past eleven years training with a mysterious woman named Karen. When he sets out to visit his older brother Kotaro Takamiya at a scientific conference, the family reunion never happens: a woman named Aramis kills everyone at the conference and kidnaps Kotaro. On his way to rescue his brother, Teppei comes across B't X, a broken mechanical horse that revives thanks to Teppei's blood. From then on B't X serves as his aide in the effort to save Kotaro from the mysterious Machine Empire. The story is a science fiction mecha adventure built around that rescue.