The idea is for a game based around the ideas and game world of White Wolf's Vampire: The Masquerade. Players would be vampires in modern-day London, tasked with keeping the Masquerade - the veil of secrecy which keeps the existence of vampires from the human world. They also have to deal with the political fallout of a "terrorist bombing" in the heart of London which appears to have killed Anne Bowesley and other top members of the London vampire underworld. You could hunt down the people responsible for the bombing, or make the most of power vacuum, or have your own little affairs... I kind of want it to be a bit sandbox-y and find I work best as a GM if I write the meat of the story once the player group is assembled instead of beforehand.

If you don't know anything about Vampire: The Masquerade, then here's a link to a wiki: http://whitewolf.wikia.com/wiki/VTM

Basically, vampires exist but have to live in secret because they're quite few in number and if their existence was publically known, they'd be eradicated. There are other supernatural creatures (werewolves, mages, Hunters, etc) but they won't be playable. According to legend, vampires are the descendants of Caine, who was cursed by God for murdering his brother Abel and turned into a vampire after drinking the blood of Lilith. The different vampire bloodlines are like the branches of a family tree descending from Caine, with different powers and weaknesses.

Most London vampires live as part of the Camarilla, a loose and semi-feudal system of government where each city operates largely independantly of each other, but obeys a few common laws (such as the Masquerade). Each city is ruled by a "Prince", who is advised by a council usually composed of city's senior members of each bloodline. However, London's last Prince - Mithras - went missing during the Blitz and his deputy, Anne, ruled in his stead for decades until she was killed in the firebombing along with most of her council. The Camarilla do not believe in the literal existence of Caine, or believe that if he did exist he is likely long-dead.

Their rival organization, the Sabbat, do believe Caine exists and that he lives to this day. They believe they will be his army when he returns to end the world and have a strong foothold in mainland Europe - especially Eastern Europe - but they are encroaching into Camarillla territory. Are their movements into London proof that they were behind the firebombing, or are they just taking advantage of a fortuitous situation?