In the final stages of World War II, a chemical warfare unit of the United States Army was positioned in the Bavarian Alps. They were send in battle against a Schutzstaffel (SS) unit, and seemingly disappeared. Both units were declared missing-in-action.
Three decades later, investigators look into rumors about zombified soldiers in the Bavarian Alps. After several investigators turn up dead, the Central Intelligence Agency (CIA) takes an interest in the case. Agent Nick Monroe is assigned to investigate the case.
He soon learns that the soldiers were exposed to a nerve gas, called "Gamma 693", that mutated them into flesh-eating zombies. The same nerve gas has kept them alive in the intervening years. Monroe also uncovers evidence of a deeper conspiracy.