The Xenosaga franchise is a sub-series of the wider Xeno franchise, along with Xenogears, Xenoblade Chronicles, and the upcoming Xenoblade Chronicles X. Made up of a three-episode trilogy, Xenosaga is set in the same universe as Xenogears and is inspired by the same Gnostic philosophies and concepts that drove the original PlayStation title. The Xenosaga series was originally planned out as a six-game story arc, but due to various factors was reduced to a three-part narrative.
Xenosaga tells a science fiction tale set in humanity's distant future, long after the human race leaves Earth for the stars. Its narrative is spread across a large cast and delves into a variety of subplots.
Each game in the trilogy features a subtitle derived from three of the major works from the nineteenth century philosopher Friedrich Wilhem Nietzsche, with which the games share some of their themes and ideas. Central to the games are biblical references, and the plot sometimes tarries with the notion of God and revolves around the destiny of humanity. Also, the question of humanity itself--what is it?--is realized through the existence and development of KOS-MOS.
Another notable fact is that the games take their names from Nietzsche's work in reversed chronological order. Friedrich Nietzsche first published Also Sprach Zarathustra and then Jenseits von Gut und Böse. Der Wille zur Macht, the subtitle of Xenosaga Episode I, was published after Nietzsche's death without his consent, and it has long been established that it is not the form in which he intended the work to be published. Conversely, Nietzsche considered Also Sprach Zarathustra, the subtitle for Xenosaga Episode III, his most important work.
Episodes I, II, and III were all released in Japan and North America. However, in PAL territories, only Episode II was released. Because customers in PAL-territories did not have regular access to the first game in the series and thus would not know the events that occurred prior to Episode II, Sony and Namco released the title as a 'Limited Collectors Edition' that contained an additional DVD with all the cutscenes from the first game. The DVD has a play time of some three hours.