There are two strands to this book: in the first, we are introduced to explorer John Hackett in the late 22nd century, about to set out on a relativistic flight to the Andromeda galaxy. On his return, five million years will have passed. That journey, in turn, sets off other lines of enquiry and Hackett continues explorations into the deep future and the way that an evolving human race copes with cosmological change.

In the second strand, we meet Mela, one of a pair of twins growing up in a roughly pre-industrial world. At night, they can see many inhabited Earths in the sky, against a backdrop of red stars. But Mela's world is slowly being overwhelmed by a phenomenon, the Tide, which is eating away at the land. And by the time Mela is a grown woman, there will be no Earth left.

The puzzle of this book is this: what connects these two stories? As Hackett's understanding of the cosmos grows, we begin to see hints that the two stories are connected.

The characterisation is reasonably done, though Hackett spends a lot of his time reacting to new wonders and being a bit curmudgeonly because the people of 5 Million AD don't react to his one-liners and cultural references (most of which are 20th/21st century). He meets and forms friendships with people of the eras he finds himself in, but most of them are fairly broadly drawn and only towards the end of the novel do they begin to react to Hackett as a person. Mela, her family and the people of the Thousand Earths are better characterised, mainly because they have a rich environment to interact with each other in. Of a bit more concern are the expository lumps: Hackett's story increasingly becomes dependant on the reader understanding some cosmology, though that only really becomes a stumbling block in the last third of the novel.

But this is definitely wide-screen science fiction of a sort that's getting harder to find these days. ( )