After Mega Man 8, Dr. Wily's whereabouts were still unknown, and he built a new castle to prepare his comeback. But a robot calling himself King appeared planning to conquer the Earth, believed that robots were not meant to simply serve humans, but rather that they were superior and therefore should rule the Earth. King ousted Dr. Wily from his own new castle, and he stole the data of robots created by Dr. Wily and Dr. Light to form his own robot army. To stop King, Dr. Wily and Light made a truce and sent Mega Man and Bass to put a stop to King's rebellion. When King attacked a robot museum to gather its data for his army, Proto Man attempted to stop him, but King cut him in half with his axe and he is sent to Dr. Light's Laboratory for repairs. Mega Man/Bass confronted King, but he withdrew, claiming not to have time for them, and left the Green Devil to take care of them. Mega Man and Bass set out to put a stop to King's plans.