Teppei is set to visit his older brother, Kotaro Takamiya, at a scientific conference after spending the past eleven years training under the enigmatic woman Karen. His plans for a family reunion are abruptly derailed when Aramis unleashes a massacre at the conference, killing everyone in an attempt to kidnap Kotaro. While racing to save his brother, Teppei encounters B't X, a broken mechanical horse that comes to life through Teppei’s blood. Now allied with Teppei, B't X becomes his aide as they confront the shadowy Machine Empire to rescue Kotaro.