From the Game's Introduction:
The world that we can see is not the only world. Some can see into that other world, can touch and be touched by the creatures that live within its walls. We call them insane, visionaries, individuals considered touched by the hand of God. These people end up in our psychiatric institutions or roam the streets, driven mad by what they see and cannot understand - the place called the Shadows.
The Shadows is a world parallel to our own, more a state of being than a physical place. Its inhabitants are trapped there, moving through the Real world but incapable of interacting with it. All items in our world; cars, people, doors, even pieces of paper cannot be affected by those in the Shadows. It is as if they move through a world populated by mobile statues, every item an immovable weight. In the Shadows, a simple rainstorm can bruise and break flesh and hailstones drops bullets of ice that rip flesh without mercy.
It is perpetually cold there, the sun never warming the streets, while all around walk in the sunshine, not feeling the eyes that watch with inhuman longing. The world invisible to the Sane is populated by the Hidden; evil creatures, sinful humans and lost souls looking for redemption, all prisoners jealous of those who inhabit the real world. The only way out for them is to become Real and the easiest way to become Real is to kill.
The inhabitants of the Shadows have many forms. Some are grotesque creatures with chitinous skin and spider‘s legs, with eyes and teeth in greater numbers than is normal. Some fly through the air over our heads, others live in the sewers below our feet. The most dangerous ones are those that walk amongst us.
All these creatures desire to enter our world and become Real. The more Real they become, the more human they are in appearance. Most of the Hidden are a long way from true Reality. They might possess the right number of limbs but details such as inch-long razor sharp teeth, or spider-like fingers, limbs that bend the wrong way or a lack of skin will always give them away. These remain in the Shadows, hiding from the gaze of normal humans, only revealing themselves to their intended victims.
Those close to becoming Real are indistinguishable from those who walk in the sun. They move with impunity through normal society, choosing their victims with care and love. They gather needy humans to their hearts; the dispossessed, the lost - those people in need of something to believe in. Some are drawn by a desire to belong, others come in search of power and end up their puppets.
The majority of humanity is blind to the Hidden in the Shadows. They move through their everyday life unaware of their presence. They cannot see the thing which sits on their dinner table looking at them with nothing but hatred in its eyes. Humans are separated into Believers and Unbelievers. The Believers are people sensitive to the possibility of the unreal while Unbelievers exist only for the material world.
These are the men and women that can see the world as it really is. Able to alter their flesh and directly affect the minds of those around them, each believe in a truth which makes the world consider them mad. Most end up committed by their family or by the legal system and spend months, or even years, undergoing drug therapy. Those eventually released find themselves pariahs, unable to get a job and struggling to make sense of a reborn world. Some end up living on the streets, homeless alcoholics or drug-takers, alone and hunted. Others, those with a supportive family or of independent means, begin to search for knowledge and eventually seek out others like themselves.
The Real World
Heretics is set in the modern day UK. The players play the part of the Heretics, both feed-prey and mortal enemy of the Hidden. They must stop the Hidden in their attempts to become Real and learn to live with an Insanity which grows in power with each passing day. They live in a world that normal people cannot know exists, a world where killers walk the streets at night looking for their victims and cults obtain victims to satisfy the hungers of their Hidden masters. In the sewers, rat-things scurry through the filth while in the shiny office blocks, spiders the size of plates move leisurely across the ceilings spinning webs large enough to catch a man. They discover that it can mean death to look anyone in the eye and the man that shouts at you from across the street might be a killer looking for a Believer.
In this world, serial killers roam the streets and the police can never find them. The dead go missing from the morgue and turn up incinerated behind the wheels of crashed automobiles. Graveyard desecrations happen every week and no-one cares that the flesh of their ancestors is being corrupted. Things that can run as fast as dogs with the chitinous skin of insects hunt in the streets and parks, chasing their prey in packs. Hideous bargains are struck between lovers and the dead have no secrets.
Welcome to Asylum Earth.