In the post-apocolyptic world of the United States northwest, a drifter trades entertainment for room and board in villages of survivors. Set nearly 20 years after WWIII (also called the Doomwar), Gordon Krantz takes shelter in a rusted out mail truck. He takes the coat to stay warm on his journey, and develops an idea. Instead of badly-reenacted Shakespeare plays, he would claim to be a mail carrier from the Restored United States.
His marks respond in a way that surprises him. They begin writing letters to family and friends that they haven't seen in nearly two decades. Gordon gives no promises, but does deliver the letters to keep up his charade. This action has a side-effect that Gordon misses, at least at first. He has given the people hope that the world is getting better.
If you have seen the movie, but have not read the book, you would be surprised. The two have very little in common. I do like the movie, but the book is so much better. And more believable.