World Of The Nightmare
In this reality the Cold War erupted into a full-on nuclear exchange in 1964, culminating in the detonation by the Soviet Union of several 100-megaton cobalt-jacketed bombs that shrouded the world in deadly fallout. The only survivors are those who went deep underground beneath the Rocky Mountains into specially prepared shelters*, and that's where they've been for the past fifty-five years, cooling their heels and waiting for the radiation to diminish to safer levels. Quarters have gotten cramped over the decades, and tensions have gotten rather high as a result.
But that's not the worst thing this world has to offer. The souls of all those billions of men, women and children who died by violence still walk this world, and their greatest desire is to inflict their eternal agony upon the living. While there are no flesh-eating walking corpses, the wailing of the spirits can be heard by anyone who ventures into the ruined world aboveground, and their cries slowly but surely drive anyone to madness. Recently the psychic force of all those tormented spirits has begun to weaken the barriers between dimensions, and soon those millions of souls could find themselves upon another world, ready and eager to inflict their eternal hatred upon the living.
*There are also shelters deep underground in Australia, but the shelters in America have no contact with them--or indeed, know they exist.