What is the plot of "By the Waters of Babylon"?

John, a young man destined to be a priest in his village, goes on a quest to the Place of the Gods to gain knowledge.  Even though it is forbidden by the priests of his village to go there, his dreams have told him that he must.  After traveling through the woods and avoiding the enemy Forest People, he comes to the Oudison River and struggles to cross it.  Once he reaches the Place of the Gods, he finds that it is in ruins.  After exploring the god roads and going into one of the buildings, John has a vision in which he sees that those who once lived in this place were men, not gods, and that they were destroyed by fire that fell from the sky.  In actuality, this is New York City in the far distant future, and it has been destroyed by bombs.  John's people are decendants of the survivors of the blast, and their stories speak of "the ground that burns forever" and warn children that if they were to touch metal they would die.  This is because those in the past had picked up metal holding radiation and died from radiation poisoning.  Many years have passed, but the tales that holding metal will kill a person still survive.  Once John learns that the gods were men, he returns home where his father warns him not to tell his people the truth.  Too much truth all at once could destroy their society.