The story is about how computation is supreme, and how reality might be a construct that is very different from what we think it is. The crazy idea in this book is that a simulation might continue running, if it is sufficiently complex enough, even if the hardware that it is running on is shut off. Around this idea this book weaves a beautiful story, of how people design worlds to escacpe into and out of them.
And how a true "metaverse" comes into existence and the problems it faces as another part of the computational universe the people inhabit gathers more complexity, thereby competing for "reality" within that computational simulation.
But the general idea, of a computation that is running, independent of a substrate that it is running in, is truly the best part of this book. It's simply mind bending, in true Greg Egan style. Just like the idea in Blood Music is mindbending and utterly beautiful.