Continuation of Teppei's journey with B't X to rescue his brother Kotaro Takamiya from the Machine Empire. The OAV series picks off from the TV series from the point where Hokuto successfully saves both Teppei and B't X and makes B't X far more powerful than before. (Source: ANN)

The Earth lies darkened and deserted. Despite the best efforts of the film's heroine, Hikaru, the evil-filled castle of Cephiro has taken over the world and stands in the center of a lifeless Tokyo. Before the ice-witch Alcyone can take her revenge on Hikaru, she is rescued by Lantis, a young man who seals her into a magical maze where she is challenged to prove her powers while her friends are hunted down by Cephiro's warlocks. Fleeing through the silent city, Umi and Fuu must seek out the sleeping Mashin; godlike creatures that will fuse with them to form giant warriors of awesome destructive power. But the Magic Knights are afraid, untrained and alone... will even the Mashin be enough to save them from the evil acts of Cephiro? (Source: AniDB)

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.