When the first reports of the Chaos Plague begin to appear in the fringe settlements of the Empire, they are dismissed as fanciful tales conjured to frighten away unwanted visitors. When the plague appears in the streets of Altdorf, however, the situation changes completely. By the time the Emperor issues his first quarantine order, the vile disease has already spread across most of his lands. The Empire's doctors work day and night to combat the sickness, but the plague defies all efforts at a cure.

Victims of the Chaos Plague first experience tiredness, aching, soreness and extreme thirst. Bouts of nausea follow, but it takes nearly a month to reach the final stages of the disease. It is only then that the horrific nature of the plague is made clear; those unfortunate souls who have experienced plague symptoms for three to four weeks begin to change, transforming into fiendish Chaos mutants that savagely attack any living creature that they see. Those lucky enough to avoid contracting the plague are often killed by the savage monsters spawned from it.

With each passing month, the death toll mounts. Quarantines and martial law fail to slow the spread of the disease, and paranoid fear grips every village and town within the Empire's borders. Militiamen in plague-wracked cities are forced to combat the hordes of Chaos mutants roaming their streets, leaving their walls undefended against threats from without. Emboldened gangs of brigands roam the countryside, looting, burning and killing at will. The scope of the tragedy is almost unparalleled in the history of the Empire, but the Chaos Plague is merely a precursor to a much greater evil yet to come.