There was once a time where the people of earth thought to be alone in the deep void of space. One fateful day, the earth was bathed in a bright green light, which rotted away nearly everything due to the radiation that came with it. The radiation was so potent that it reached nearly every part of the earth, effectively killing 3/4 of all life on the planet. Even after 4 years have passed, the damage was staggering; most of the earth was reduced to a toxic wasteland, but luckily, there were survivors.
One survivor in particular (which we'll nickname Gammafreak) had been mutated along with several surviving animals and humans. As of now, Gammafreak has the ability to manipulate energy, gamma energy being the most comfortable to use. Gammafreak has been aproached by a group of survivors that have noticed his powers, and have shared information on what caused the attack -- An alien race.
They elaborate on there being hyper-advanced machinery in the ship, including a time machine. They know that there is a time machine on the ship because of one of the survivor's ability. The only way to save the earth is to go back in time and redirect the radiation back at the alien ship, which will destroy the ship and the aliens that were on it. Unfortunately, the reason why the aliens have attacked the earth is unknown.