King took over Wily's new castle to use as base and managed to recruit eight robots to help him, either voluntarily or by force, two being Dr. Wily's robots Astro Man and Tengu Man. He attacked the Robot Museum and stole the data of the battle robots to create a robot army. Proto Man tried to stop him, but he was defeated in one hit. King left the Green Devil to cover his escape. King is later on fought at the second King Stage as the penultimate boss. At first, he seemed unbeatable since he had a reflective shield that would send Mega Man's and/or Bass' shots right back to them. However, the newly repaired Proto Man warps in and destroys his shield with his "Big Bang Strike" attack, leaving King vulnerable. After that, Mega Man and/or Bass were able to engage King in combat once more and defeat him. Defeated and weakened, King asks the heroes why they go so far as to fight for "lowly beings" like the humans, and then tells them that robots are "more advanced" than humans. Mega Man responds by telling him that neither of them are better or more advanced than the other (though this is only one interpretation of what he actually says, which is something along the lines of "Superiority isn't relevant to this issue!"), while Bass, on the other hand, tells him not to forget that humans are the ones who created robots in the first place. After that, King tells the heroes to finish him off, but not before warping the exhausted Proto Man out of the area. Mega Man then tells King to escape from the castle with him, and that Dr. Light will be glad to repair him. King then responds by telling Mega Man that it won't work, because his body is set so that it'll explode if anyone other than Dr. Wily tries to tamper with it. In Bass's story, he'll just tell King that he's leaving, and will ask King who his creator is. He, of course, responds by telling him that it was Dr. Wily.
In both stories, Dr. Wily will appear on the monitor, scolding King for siding with the enemy, as well as telling him to finish the two heroes off. King ignores him and instead asks again why robots fight for humans. Then, Dr. Wily uses a device to boost King's "brainwashing" level. After that, King flees into the next room, and then Mega Man and/or Bass will chase him. After that, King merges with King Tank and King Plane into a big tank-like robot, Jet King Robo, and engaged the heroes in combat once more. After King's defeated again, he changes back to normal, telling the heroes to get out of his castle quickly before it explodes. First, they ask about him, but he responds by saying that he'll make it out of there after they do. Once the hero teleports out, King laments that he doesn't have a transporter unit himself. In Bass' story, he says that he and Bass could have been good friends. In Mega Man's story, he'll say not to let any more robots to suffer his sad fate. After King's "last" words, the castle exploded.
After Mega Man returned home from his battle against Dr. Wily, he was saddened by the fact that King supposedly died, but Roll gives him a letter with King's insignia on it (meaning that it was obviously from him, which confirms that he survived the explosion), and it turns out to be a letter thanking Mega Man for helping him realize the error of his ways, apologizing for what he's done. He then says that if they are to meet again, he hopes that it'll be as friends and not as enemies. In Bass' story, Dr. Wily revealed plans for a "King II", offering Bass a chance to team up with him when he's complete. He then tells Bass that if he joins with King II, the two of them will be an unstoppable duo. Before Bass could answer, however, Proto Man warps in and destroys the blueprints, foiling Dr. Wily's plans once more.