It started during the days of the Second World War, as HYDRA, the Third Reich's deep science unit, waged war on the Allies and their champion, Captain America. A top secret weapon developed by the US government, Captain Rogers was lost over the Arctic saving the world.
Then came the quartet of friends who went to the stars. Dr. Reed Richards and his friend and co-pilot Ben Grimm, along with Reed's girlfriend Susan Storm and her brother, Johnny, passed through a belt of cosmic rays and returned to Earth changed. Granted fantastic powers by their trip, they became known as the Fantastic Four, the first celebrity superheroes.
Soon thereafter, the Children of the Atom emerged from generations of secrecy. The Xavier Institute trained young mutants in the use of powers they had been gifted with at birth. While feared and hated by the public at large, they nonetheless are some of humanity's greatest protectors.
The criminal underworld in New York City is ruled by an iron fist inside a velvet glove, controlled by a woman known only as the Queenpin. HYDRA, long believed inconsequential, has once again begun to openly plague the world.
In response to HYDRA, SHIELD has been formed as a global watchdog and response organization. Led by Colonel Fury, they have begun to contact various superhumans and mutants for Fury's Avengers Initiative.