There was a virus outbreak, and it caused some... issues. Many people were dead within days, but those who weren't killed transformed into beastly creatures. Those who showed signs of contamination were quarantined in the Death Dome, where they grew into "The Behemoths."
That's the premise of the Glu Mobile subsidiary Griptonite's new game, Death Dome. You play the part of Phoenix, a fearless protagonist set on slaying the mutated monsters inside the Dome.