Xenogears follows protagonist Fei Fong Wong and several others as they journey to uncover the truth behind mysterious, cabalistic entities operating in their world. The principles and philosophies of Friedrich Nietzsche, Sigmund Freud and Carl Jung influence the plot, character design, and world of Xenogears. Additionally, the symbols, theological concepts, and devotional practices of several world religions are represented in fictionalized forms in the game. Major psychological themes are the nature of identity and human memory, particularly as these relate to the phenomenon of dissociative identity disorder. The relationship between humanity and machines is central to the game's plot, as indicated by the presence of giant robots dubbed "gears," which almost each playable character can control.