From the introduction:
Once upon a time, not too long ago, a handful of scientists, explorers and science-heroes – let’s call them ‘Reality-Nauts’ – found a way to access the Multiverse. They found an alternate Earth, very much like their own, but different in interesting ways.
These Reality-Nauts soon discovered that this Earth had its own explorers, scientists and science heroes studying the idea of the Multiverse and attempting to open a portal. However, no-one on this Earth had managed it yet. Reality-hopping is the one single exceptional piece of technology that the people of this alternate Earth had not yet worked out for themselves.
But the greatest surprise was this: just as one team of Reality-Nauts stepped out onto this alternate Earth, so did four other groups of Reality-Nauts from four other worlds! A small handful of Earths – independent of each other – developed their own means of transporting themselves across realities, and had converged on this one reality at the same, exact moment.