Horzine Biotech, a company specializing in biotechnology, was tasked by the government to perform cloning and mutation experiments on human subjects. When something goes terribly awry and the specimens on which the experiments were performed become insanely hostile, the company and its military protectors are overwhelmed, allowing the specimens (referred to as "zeds") to spill into the cities of London and devour its citizens and law enforcement.
Led by Dr. Kevin Clamely, who has since been cybernetically enhanced, the zeds have ravaged London and are intended to expand into overseas territory. In attempts to contain the situation, the military has dispatched eradication teams into the infested areas whose task is to wipe out as many of the specimens as possible and ultimately take out their leader, the patriarch. You the player are part of one of said teams and will have to work strategically to eliminate the threats while staying alive!