At last, the Gunpla Battle event known as "GB Fest" has opened. The venue is filled with people, and a long line forms at the booth for its main attraction, a special event battle. In the line are Ryusei, Sana, and Touma, each of them with high expectations in their hearts as they prepare to test the strength of the new Gunpla they have built.

During the event battle at GB Fest, the sudden appearance of a masked man throws the venue into an uproar. The Gunpla are rendered helpless by the mysterious "Runner Prison" system this man controls. Ryusei, Sana, and Touma manage to avoid it, but then the masked man unleashes an even more astonishing move against the trio.

After a hopeless struggle, Ryusei and the others have been captured by the Runner Prison. Now the masked man finally summons his own Gunpla, the Gundam Barbataurus. His three opponents are in desperate peril as they face this demonic machine that tramples the battlefield with overwhelming power.

The identity of the masked man is revealed to be Mahara, who was defeated by Takuma in the final round of the Gunpla Battle U.S. Championship. With Misa's assistance, Ryusei and the others escape their predicament, then do their utmost to confront Gunpla controlled by the special AI characters that Mahara has created.