There are some current elements, too, like people being harassed when they cross borders, and the president of France allowing, encouraging even, young black-clad jackbooted men to kill minorities.
The book starts in 1959 with a death and our first protagonist, Wendell Floyd, getting the case from her landlord. The police think she jumped off her balcony. Then we move to the frozen Earth of 2266 where Verity Augur is trying to get an artifact from before the Nanocaust. We move back and forth between our Earth and Earth 2 through most of the book.
It turns out that the woman who was killed in E2 was sending archeological materials back to Earth via a hyperweb -- a wide range of interspace tunnels that were created by someone/thing many centuries back and that we don't really know as much about as we should. Earth sends Augur to E2 to pick up some of the artifacts that they think are important, but the landlord is then killed, too. Floyd thinks Augur is a spy, but is also trying to figure out what she's a spy for, and ends up going to Earth in the last use of that tunnel. While they're in the tunnel, she tells Floyd, for many pages, what the Nanocaust was.
They find that the baseline humans (Threshers) and the aggressive portion of the changed humans (Slashers) are fighting a war both on Earth and in space, and that one of the aggressive Slashers wants to use Silver Rain (a nanovirus that kills everybody) on E2 to clear it of people and then take it over themselves. Floyd and Augur work with some moderate Slashers to try to keep the Silver Rain from being delivered.
The source for the address of E2 is something I figured out right away, but isn't figured out by Floyd until near the end of the book. I always wonder if the authors put something like that in to make me feel smarter.
After I was over the confusion about the shared elements, I liked the book, but found it pretty predictable. I'm going to take a break from the Reynolds books and reread Hogfather next.