The player characters are all full-time members of the Exemplars, a legal superhero organisation based in San Francisco and dedicated to upholding the public good. The Exemplars were formed by the merger of the San Francisco X-Men (a financially independent team with substantial assets and full-time members on salaries) with the Champions (an irregular team comprising all active Los Angeles superheroes, mostly part-time and self-funded) on 11 June 1998. It is one of the most prominent and powerful superhero teams in the United States of America, along with the Avengers in New York and the Guardians of Liberty in Dallas.
As a significant number of the current Exemplars were members of the San Francisco X-Men, the Exemplars continue to publicly support the Xavier Vision and to oppose terrorism inspired by genetic differences. At its simplest, the Xavier Vision is to work towards the peaceful, integrated co-existence of mutants and non-mutants on this world. It is attributed to Professor Charles Xavier, the famous geneticist and mutant activist who was "outed" as a mutant by investigative journalist Lucas Hamble on 20 February 1991. Other organisations supporting the Xavier Vision are X-Works (a corporate body dedicated to promoting the issues of mutant rights and mutant equality) and Xavier's School (a private school dedicated to providing educational opportunities for mutant youths). The Xavier Vision stands in contrast to the "post-human" society of Genosha, the "mutant homeland" in the Indian Ocean created when mutants occupied the island of Diego Garcia on 22 November 1994. Mutants comprise over 90% of the population of Genosha, a rogue state with limited international recognition and ruled by its elected president, Magneto.
In general terms, the campaign world is an alternate version of the present-day Marvel Universe. The campaign departed from the comics in Uncanny X-Men #180 (published April 1984): the odd machine in Central Park was a trap set by Arcade for the X-Men, and not the Beyonder's portal to take the X-Men the Secret Wars. Anything that happened to the Marvel Universe in a comic published before or at the same time as Uncanny X-Men #180 definitely occurred in the campaign world, though not necessarily exactly as presented: for example, in the campaign world the Fantastic Four gained their powers testing an experimental spaceship for a trip to Mars, not the Moon. Anything published in later comics may or may not have happened, and may have happened differently if it did: for example, the history and location of Genosha are different in the campaign world to the published comics.
The campaign world also includes many characters from other superhero settings, including DC Comics (for example, Captain Atom), Cyclone Comics (for example, the Southern Squadron), and the Champions Setting (for example, Eurostar). Anything published about these "imports" probably didn't happen, or happened differently if it did: for example, Captain Atom still received his powers because of a US military test involving an experimental nuclear weapon in the late 1960s, but the scientists involved in the test included a youthful (pre-Hulk) Bruce Banner.
The modern age of heroes (beginning with Fantastic Four #1, published August 1961) started in 1976. The "all new, all different X-Men" (Giant-Sized X-Men #1, published May 1975) were formed to rescue the Original X-Men from Krakoa in 1980. The compressed timeline for the modern age of heroes means the events of comics published from 1961 to 1975 took place from 1976 to 1979, while the events of comics published from 1975 to 1984 took place from 1980 to 1983. The campaign has now been running for 21 years, and 16 years have passed in the campaign world. Spider-Man and the Human Torch are both approaching forty, the original Captain America is now Director Steve Rogers of SHIELD, and Franklin Richards is in his early twenties and an active member of the Fantastic Family. The Original X-Men are all in their late thirties except Jean Grey, who was brought forward eight years in time (from 1982 to 1990) by Cable instead of dying on the moon at the end of the Dark Phoenix Saga (Uncanny X-Men #138, published October 1980).