Just finished a novelette by James Patrick Kelly with the typographically difficult title of 10^16 to 1. It's in several collections, but I read it in The Secret History of Science Fiction.
We have two science elements in this plot--time travel and the "many worlds" physics hypothesis. Ray Burchard, a 12 year old boy, sees a semi-invisible thing in the woods that reveals itself to be an odd sort of "person". Burchard is fully prepared for this as he is a big SF reader. The "person" seems to be from the future (though he won't admit it at first), and holes up in the Burchards' fallout shelter, waiting for a chance to change history. In the end, it might be up to Ray.
It's an OK story as these go, though there are better ones. What interests me about the story is when it was first published--1999. The story is set in 1962, when fear of nuclear war was at it's height--the Cuban Missile Crisis. Was it our biggest fear in 1999? I don't think there are many now who would rate a nuclear exchange as the most likely way we will destroy the world. So what is Kelly getting at here? Is he going kind of meta on us, projecting 1962 fears into the far future? I don't really know. But it was a pretty good story, anyway. 3 stars