A couple on a train trip are confronted by a mysterious little man with an even more mysterious slab that can show them how things would have happened in their lives if the coincidence that brought them together never happened.
Asimov was challenged by his first wife to make up a story about their regular train trips between Boston and New York, and this is the result. The story is a fantasy, rare for Asimov, and a romance to boot, also rather rare. It works well enough, particularly as a romance—although the couple don’t meet in the “if” universe as they did in real life, they are eventually brought together by circumstance anyway—being “destined for one another” is a very nice romantic concept, after all.
In fact, this story would probably do well in an adaptation to an anthology series such as “The Twilight Zone.” Pity it wasn’t tried.
Still, it is rather atypical for Asimov, so I'm not sure that most of his fans would be enthusiastic about it. A good story, but not top-notch.