This is the story of John Rayburn, but it's also the story of another John Rayburn. As a relatively normal Ohio farmboy, John is (naturally) surprised when he meets another version of himself- himself from an alternate universe. John Rayburn number 2 (or "Prime") introduces himself, and shows John the device that has allowed him to hop across universes. He convinces John to try the device for himself, only for John to discover that the device is broken- it can go forward across the universes, but not backwards. John can't go back to his own universe. From there the book follows each version of John. The original goes from universe to universe and attempts to find a way back home. Meanwhile, "Prime" attempts to take over the life of his unwitting patsy. John eventually settles down for a while in a new universe (one where he was never born) and in the course of studying engineering (in an attempt to decipher the secrets of the device) accidentally invents pinball, which doesn't exist in that universe. Prime, meanwhile, is deliberately trying to do something similar with the Rubik's cube, which was never invented in John's universe. Each attracts attention from some mysterious other universe hoppers, who seem to have questionable motives at best. The book follows their divergent timelines through to a fascinating conclusion.
Best part of story, including ending:
I loved the mechanics it made for universe hopping. For instance, if you're standing on a stairway when you transfer, but that stairway wasn't built in the other universe, then you end up falling. It also really explored the possible uses and ramifications of universe travel- such as taking advantage of inventions like the Rubik's cube and so forth.
Best scene in story:
There was a scene where John considers doing the same thing that Prime did to him- ie, taking over a new life from another version of himself. When he starts to do it though, he finds that this version of him is missing an arm from a childhood accident that John himself almost had. It's sort of a fascinating scene in terms of what John finds himself thinking, and the guilt over what he almost tried to do.
Opinion about the main character:
Both characters are very interesting, but I found Prime to be a little too unsympathetic. I'd have liked for there to be more similarities between him and John, in order to further the parallels between them and reinforce the whole "for want of a nail" message that the book seems to have going.