Like a lot of great SF, this starts with just one single premise. What if we suddenly discovered an unlimited number of pristine Earths, and we could all travel between them at the flick of a switch?
One day, a blueprint for a very simple electrical device appears on the Internet. Thousands of people build one of these little boxes, flick the switch — and disappear. They’ve “stepped” to an alternative Earth, untouched by humans. From there they can go back or continue on to a multitude of more-or-less parallel Earths. The story follows a number of characters through the massive social changes that follow on from this: suddenly there are endless frontiers, on pristine worlds where environmental degradation and climate change are irrelevant. There is also a lot of sci-fi adventure as people explore millions of the alternative Earths, some of which are very different — essentially alien worlds. And they don’t just stop there — the third book in the series is called The Long Mars.