The story takes place in the world of "Kamen Rider BUILD" which is divided into three blocks by the sky wall. The enemy monsters from the world of "Kamen Rider EX-AID" appear unexpectedly: Why do enemies that shouldn't appear here come to this world? Facing the mysterious enemy, Zhantu has no way to deal with it. A mysterious man with half of his body covered with metal appears in front of him. The man who claims to be a scientist starts a huge machine, and another upside-down world appears in the sky. On the other side, the same situation occurs in the world of EX-AID. The world with the sky wall is gradually falling into the world of EX-AID, and there are only 24 hours left before the two worlds collide. The countdown to the end of the world begins...! Faced with an unprecedented crisis, only those warriors can save mankind. Surpassing the scale of the previous work, the action scenes have never been seen before, and the heroes who should not have intersected finally meet. The story full of mysteries is about to be solved——