The premise courtesy of Wikipedia:
Hellgate: London takes place in 2038, post-apocalyptic London. London has been invaded by demons from Hell. Up until recently, humans have had many champions looking to hold back the flood. Various real-world events are referenced in the background of the Hellgate story as averted crossover attempts. The Crusades were actually undertaken to fight back the minions of Hell, as was the charring of London in the Great Fire to wipe out the Plague. According to the story line, the famous Knights Templar were the keepers of knowledge on how to battle these demonic forces. Unfortunately, these heroes underwent a charring of their own at the hands of a jealous King Phillip IV, who hated the power they held in the world at large. While the Knights Templar survived as an organization, their numbers were severely diminished and were forced to remain in hiding to stay alive.
As time passed and technology progressed, the old ways began to fade from memory and the secrets of the arcane were lost. When the demons attacked again, they found their enemies unprepared. Ordinary weapons were no use against the demons, who could shrug off an RPG round. The Templars offered their special services to the military, but the leaders balked, refusing to believe in the old truths. Hellgate quickly took the victory in the battle for Earth. The Burn, a decades-long process of "hellforming", has begun.
However all is not lost, as various groups have been preparing for just such an eventuality. Freemasons built the London Underground to be demon resistant, and as such the stations now act as bases, "safe zones" for would-be defenders. These defenders are armed with a fusion of modern weaponry and arcane magic such as flamethrowers that throw Greek fire or swords covered in locusts. In the game, players get to be one of these defenders.
