In the year 22XX of earth, a botanist by the name of Doctor Cain stumbled upon a capsule containing a robot - the last creation of Doctor Thomas Light, and the first robot with the ability to think and make decisions for itself. Activated by Cain, X taught Cain about himself, and the elderly man began the creation of Reploids, or thinking, decision-making robots based off of X's designs and programming. It revolutionized the world, but it was not without its flaws: Reploids could choose to turn against humans, and become "Mavericks". Thus, the Maverick Hunters were born, and first among them, the most powerful reploid of his day, Sigma.
Infected with a virus during a battle with a strange red Maverick (Zero), Sigma became the leader of a Maverick rebellion that threatened to eliminate humans from the face of the earth, and replace them with reploidkind. X and Zero, who had been repaired by Cain and made a Maverick Hunter commander, fight against Sigma and his various incarnations, as well as the reploids who have become infected with the Maverick Virus, or have chosen to threaten the lives of human beings and bring destruction and war to Earth.
This series covers the canonical events of the Megaman X games 1 through 8, Command Mission, and Xtreme 1 and 2.