As summarized on the game page on the Treefrog website: "The premise is simple: The year is 1881 and the 'Old Ones' have been ruling the earth for over seven hundred years. Although most of humanity has accepted these monstrous rulers, there is a growing underground movement to overthrow the regime, labeled the Restorationists. A secret war is being fought around the cities of the Europe and the New World between agents of the Restorationists and those loyal to the powers that be." (The story is available in PDF format on Gaiman's website.) Here's a short description of the gameplay from Wallace:
Deck-building forms the core of the game. You use influence cubes to bid for the right to draft cards and take control of cities. Each player has a secret identity, either a Restorationist fighting against the creatures or a Loyalist attempting to defend the status quo. Which side you are on determines what you score points for. An additional twist is that the performance of other players on the same side as you can stop you from winning if they are doing particularly badly, so you really want to know who is on which side. More specifically, when the game ends – and this can result from multiple causes, such as a marker on the War or Revolution track hitting 15 or the assassination of a Restorationist player agent – then the sides compare their scores; which side has the lowest score automatically loses, then the player with the highest score on the remaining team wins the game.
The game takes around ninety minutes to play and is more complex than most, so more suitable for the experienced gamer.
In addition to having a text only version of the rules online (PDF), Wallace has print-and-play components on the Treefrog website and a slideshow overview of the game that's prefaced with this warning: "What follows is a general description of play. For all of the nitty gritty details you will need to download the rules and try to make sense of them." Oh, that Wallace – making light of his sometimes difficult-to-follow rules. Or perhaps you'll instead be overcome by the influence of the Old Ones...