Simius is the leader of the Man Apes, a race of mutant apes with human-level inteligence. Simius and the Man Apes have long been at odds with a neighboring village in the San Fernando Valley of Los Angeles, California and with its leader, Zett. The conflict between them came to a head when Thundarr, Ariel and Ookla foiled an attack on the village by the Man Apes. Zett initially believed they were allied with the Man Apes, but quickly discovered otherwise and explained to the group that the Man Apes had been terrorizing their village for some time and were in search of the mising pieces of something Simius called "The Mighty One", which turned out to be a gigantic robotic gorilla (highly implied to be a leftover movie prop from before the Cataclysm in the image of King Kong). Simius eventually succeeded in bringing The Mighty One to life and set it against Zett and the villagers, but it was destroyed by Thundarr. The Man Apes were defeated and Simius himself was captured by Ariel and Ookla.