From the back of the box:
A shadow lies over Averheim. The rich and prosperous city is caught in the grips of a struggle for succession, its Elector Count dead these two years with no heir. Sinister forces stand poised to exploit this weakness. And now, as war breaks across the Empire's borders, a vile conspiracy gnaws at the heart of the Empire. The true enemy will soon be unmasked...
The Enemy Within is a full-length campaign for Warhammer Fantasy Roleplay. From humble beginnings in the poverty-stricken docks of Averheim, the heroes are plunged into a plot that spans the whole of the Empire, from Averheim in the south to the City of the White Wolf in the north. The heroes will endure treachery and betrayal, foul magics and fouler smells, violence and intrigue.They will see miracles and terrors. They will save the Empire, or destroy it.
GMs will find dozens of game aids within, from the clue cards detailing multi-layered mysteries to an innovative system for determining which noble champion of justice is secretly the villainous Black Cowl. Other features include unique player-character backgrounds that tie them into the story from the beginning.
There are many dangers in the Old World, but none so perilous as the Enemy Within.