The world is modeled on most science fiction mythology. The Xenosaga series began in the year T.C. 4767, and mankind exists on distant planets and artificial colonies. Earth is seemingly gone; it has been erased from starmaps and renamed "Lost Jerusalem". The capital of all known space is Fifth Jerusalem, where the Galaxy Federation keeps watch over mankind. The Xenosaga universe is not divided into galaxies and solar systems, but rather is divided into UMN columns. Although initially host to humans, science and the paranormal have given birth to several quasi-lifeforms, including androids, cyborgs, Gnosis, Realians, and U.R.T.V.
Besides the main story arc, various characters are featured heavily in sub-plots. Shion Uzuki's arc involves her overcoming the tragedies in her past, including the brutal deaths of her parents and lover, which she witnessed as a child. The other arc involves Gaignun Kukai, Jr., and his anger with his twin brother, Albedo, as well as his father, Dmitri Yuriev. Episode I primarily deals with Shion's arc, while Episode II primarily deals with Jr.'s. Episode III brings a close to both arcs, in addition to the main story.
