Master of the Robot Kingdom, Roger Smith, has it all. Big O is at his command, an army of Big-4 are behind him, and the negotiator extraordinaire that he is, every job ends with a good payout for Smith and his employer. There’s just one problem: Big Fau won’t be ready in time to combat the threat to Paradigm City.
Big Fau is Big O’s backup, a vast amount of Big-4 technology in the form of a giant robot that automatically engages in battle when Big O is destroyed. Suddenly, Roger has no choice but to entrust Big O with the job of taking down this new enemy.