From the campaign:
In The Shadow Over Westminster, each player assumes the role of one of the Agents: the secretive Illuminati Shadow, the noble Knight Templar, the perceptive Watcher or the mysterious Outsider. As a team, you must build up your skills by acquiring advanced investigative techniques and powerful artifacts in order to deal with the strange disturbances that are appearing around the city.
Successfully unraveling the mysteries provides clues to the source of the darkness, and when enough have been studied, the Cataclysm will be revealed. Will it be the Ancient One that sleeps deep in the bowels of the Earth? Mysterious beacons signalling an alien invasion? A devastating plague manufactured in some underground lab? The very gate to Hell itself? Or perhaps one Agent has been playing the others as fools in their own dark scheme. Whatever it is, will you be able to stop it in time?