A number of Reploids turned against humans. These Reploids were dubbed "Mavericks" ("Irregulars" in Japan), and a force called the Maverick Hunters ("Irregular Hunters") was formed to combat them. The Maverick Hunters were led by Sigma until he, too, became a Maverick and declared war against the humans, thus starting the Maverick War. X took it upon himself to join the Maverick Hunters under their new leader Zero. Throughout the series, X, Zero, and later Axl (a prototype of the next-generation Reploid) battle against the Mavericks to stop their plots to destroy the human race.