Everyone wants to cast spells and control reality with their minds, right? Problem is, just like in Mage: The Ascension, reality is consensual, and the consensus is that there's no such thing as magic, ghosts and gods are pretend, and the world is scientific, rational, and predictable.
Well fuck that, say a bunch of total assholes, I want to make fire with my mind!
So they form a conspiracy to basically go around committing atrocities and planting fake stories in the media and staging hoaxes to try and get people to believe that the world is in fact random, unpredictable, and occult. If they can convince enough people that that's how the world works, they can have all the magical powers they dreamed of when they played D&D! The fact that this involves killing lots and lots of people and making the world a genuinely shitty place to live for everybody but themselves doesn't bother them at all, because the Esoterrorists are sociopathic monsters. Well-funded and highly skilled sociopathic monsters, for the most part, but sociopathic monsters none the less. It helps that, again, magic isn't real, and so the most they can usually do is find a local legend about a monster or something and kill a bunch of people in a way that looks like the monster did it. Once in awhile they can summon a monster or something, but this tends to fail seeing as how, again, magic's not real.
The player characters are a bunch of members of a secret society that goes around investigating possible cases of Esoterror and capturing or killing the individuals responsible.