A well-written book with a fascinating plot which,unfortunately, sputters out with an implausible ending.

The characters are a couple, deeply in love, and caught up in every-changing multiple-universes. In one universe, airplanes have not been invented and flight is via air-ship dirigibles. In other, there are airplanes. Sometimes, even though their surrounding appear the same, they know that the universe they are in has shifted because they look up in the sky and see an implausible flying vehicle. Other times the shift is dramatic and unmistakable. Each shift throws their lives into turmoil: Money they have painstakingly saved from low-wage labor suddenly has the wrong presidents on the bills, because the history of the new universe is different from the one they have just left. Even worse, because they have been holding hands or touching at each shift, they are in constant fear that if a shift should occur when they are no touching, they may end up in separate universes, forever torn apart.

Their relationship is a great love story: Two people with very different -- actually contradictory -- religious outlooks being drawn powerfully together, totally devoted. The underlying themes about human relationships -- the real basis of love -- make this book thought-provoking. It is a book which you think about when you are not reading it, and which you want to get back to.

The explanation in the final chapters about what was happening to them is totally contradictory with the facts which had emerged in the story. However, the unfolding of the reason why these things had happened, does make sense. So you sort of have to suspend your skepticism of the plot, and just allow yourself to focus on the theme as being more important. But you really should not have to do that. Even if the human drama, and the psychology of the characters is of primary importance in a mystery, nonetheless the reader has a right to expect that there will be a genuine solution to the mystery in the end. As I read the book and saw how a plausible solution seemed to be becoming increasingly impossible, I anticipated that Heinlein would come up with an ingenious resolution in the end. He did not quite resort to the old, "it turned out to be a dream," but the explanation that he did resort to made so little sense as to be just as disappointing.