King was created by Dr. Wily as a very powerful robot, intended to be a test for Bass, because Wily couldn't understand why Bass couldn't beat Mega Man, even being more powerful than him. However, King betrayed Wily and steals his plans for 8 new Robot Masters, later creating his own army of robots. He soon proclaims himself the King of Robots, and decides he will create a perfect world for the robots.
He invades Dr. Light's lab and steals some of his projects, so Light sent Mega Man after him, at the same time that Wily sent Bass to destroy him. Each of them go their own way after King, Mega Man to prove that King is wrong on his way and Bass to prove that he is the strongest. Protoman also goes after him, but is severely injured. Mega Man and Bass both defeat King's army, making their way into his fortress and engage him. Despite their best efforts, King manages to block all of their attacks with his sheild, until Protoman appears and uses all of his power, managing to destroy it.
King is defeated and then reveal that it was Wily who created him. Wily then brainwashes him to fight again, this time using a giant mech armor. As he is defeated, he snaps out of his brainwashing and helps Mega Man and Bass to flee his fortress, which starts to explode. Later on, Megaman receives a letter from King, revealing that he is still alive and saying that he was wrong and decides to help build a world that is good for both humans and robots. Bass defeats Wily, who reveals that King was meant to test his capabilities, and also that he had plans for a new King, but Protoman arrives at his lab and destroy the plans.