Are you familiar with the Necessary Evil campaign setting? Here's the introduction from the players part of the campaign book.
Setting the Stage
Of course it started on January 1st, 2001. The first ships appeared over every major city of the planet. There was no prelude and no demands. The aliens just attacked. They were an amphibious race that combined the nastiest aspects of sharks and crocodiles with an aggression matched by neither. They were the stuff of nightmares to most. The people called them “Fins.” New York became a war zone, Tokyo burned, and even sunken Atlantis was rubbled and its ruler—the legendary Aquarian—slain. Earth’s superheroes fought back immediately. The fins were terrifying, but no one ever doubted the heroes would win out in the end. After all, aliens had tried to invade Earth three times before, and earth’s defenders had always emerged triumphant. But the fins possessed advanced technology and savage bloodlust, so the losses were heavy. Earth began to worry.
Then came the turn of the tide the people had come to expect in these epic wars of aliens and supermen. Advance forces of another alien race arrived in earth orbit—the V’sori. They claimed to be the long-lost allies of Atlantis who had been searching for their missing comrades in arms for hundreds of years. During their quest, they had grown to become the defenders of the galaxy.
The V’sori claimed the Fins—or K’tharen as they called themselves—were rampaging mercenaries who ravaged world after world, but the V’sori had fought them before and would help Earth against them now. The world’s greatest superhero, Champion of Alpha Force, flew into space to meet them. He returned later and gave a stirring speech that called for humanity to hold out for just a bit longer. They did, though the final cost in blood was great.
When the V’sori arrived, the K’tharen fleet rose to fight them. The alien ships fought a titanic battle in space, but the V’sori won out, and even beamed the images of the fleeing Fins to earth. Humanity cheered their saviours and welcomed the advanced alien protectors with open arms. Anxious youths danced in the streets, proclaiming an era of peace and love. Less trusting commentators were booed and mocked in the national press. And some appeasement-minded governments even offered the aliens the figurative keys to their states.
Of course that’s just what the aliens wanted all along. In every major city, heroes and world leaders joined together to welcome the aliens. The crowds cheered as the V’sori ships appeared overhead. Every superhero, sidekick, president, and even stars of sports, movies, and music attended these grand galas. The people of earth cheered as they watched their heroes greet the alien saviors on the news networks. The cheers turned to screams when the V’sori ships opened fire on the gathered throngs. Moments afterward, the K’tharen warships returned—servants of the V’sori.
Champion was standing before the cameras when the V’sori hit. They knew his weakness—radiation—and targeted him with a concentrated beam of energy that turned the hero into a pulpy mass. Massive death rays then swept over the gathered throngs, slaying thousands of heroes, officials, generals, and innocents in moments. Within 30 minutes, the unthinkable had occurred. Earth’s greatest heroes were dead and its leaders vaporized.
The aliens had won.
Soon, televisions and radios around the world carried an announcement from the admiral of the V’sori fleet. Earth was now under the control of the V’sori Empire. Resistance would be met with death. The V’sori Empire wanted Earth’s resources. They needed a way to study our defenses, and then take them out with a surprise strike of surgical precision.
They succeeded flawlessly. They claimed destroying more of our infrastructure and our people was counterproductive to their goal, gaining a new selfsufficient source of raw resources. The V’sori have placed Governors in charge of the former nations of Earth. The invaders and their Fin servants are now commonplace. There isn’t a single government left to oppose them, and they feel they have nothing to fear from Earth.
But they are wrong.
Earth’s heroes are dead, but there are other beings with extraordinary abilities on our planet. They are supervillains. And they are a necessary evil.
The year is 2004. The Earth has been under the control of the V’sori Empire for over two years. Humans with extraordinary abilities, commonly known as superpowers, have existed for nearly a century. Those who used their powers for altruistic reasons were known as superheroes. It is believed somewhere around 98% of them have been killed by the V’sori.
Various resistance movements have been quashed by the V’sori, but the most promising has only just begun to act—Omega. Omega was started by the greatest criminal mastermind of the modern age, Dr. Destruction. Its members are largely made up of former superhumans the world once called villains.
Dr. Destruction has said that only he is the rightful ruler of the earth—”not some alien scum from the back corners of the galaxy.” Omega is organized into cells, each of which is only aware of the other members of its own cell to prevent betrayal. Cells are contacted with their mission information through blind drops, secret codes, and most commonly, encoded radio broadcasts.
The game would start with the players being transported to jail, after being captured by the aliens, but Dr Destruction breaks them out of the the prison transport, tells them they're now working for him / Omega and welcome to the resistance.
It's a bit of a shift from what you have as a background, but I don't think too much of a one.