The story works on the premise that there is no faster-than-light travel. As a matter of fact, when interstellar travel begins, spaceships only travel at about 10% of light speed - there's no significant relativistic time dilation to make space crews perceive long journeys as taking not as much time. Therefore, a central theme of the book deals with the intertwining of interstellar travel and a means to make humans live at a much slower pace, so what seems like years to others seems like days to these slowed-down humans.
The first quarter of the book gives us the background. A billionaire who owns and lives on a space station entices a research team working on slowing down life processes to bring their operations to the space station. They arrive just in time. The countries of the world - which have become in greater conflict as a result of the effects of climate change and limited resources - have an all-out nuclear war. Only people living in space are left. Now jump ahead 20,000+ years normal time. Some of the main characters are those we met earlier, but are still alive because they've been living at a 1/2000th slowed-down pace.
Personally, it didn't seem to me we had to spend 1/4 of the book giving the background for the main story.
The research team on slow living has been the central group for interstellar society. A number of planets have been settled and they initially developed separately. However, none were that successful. They've suffered from corrupt governments and such. After some failed attempts at interactions between the planet-living people living at normal pace and the space-living people living at slow pace, the space people have arranged mainly to harvest some of the best people from the planets to join the space-living people. Those living at slow pace can't reproduce so they are dependent on the planets for more people.
The best people on planets are selected through a series of competitions. A group of winners from on planet come to question the official story about the space people and what happens to winners of the competitions. They find ways to go off planet and investigate the truth.
...an investigation is made of apparently artificial transition of Sun-like stars to red dwarf stars. ...an expedition is sent to go to what appears to be the first of such stars. ...when they return, much has changed...
The book should be interesting to readers who would like to see ideas on how to deal with long interstellar voyage times, issues of "suspended animation" and the like, the idea of individuals living slowly to see the far future, etc. There is also the idea that red dwarf stars last much longer than Sun-like stars - so they may be desirable to civilizations planning for survival over many billions of years. Some hard SF readers will appreciate a book that does not feel it necessary to include FTL in a story about interstellar travel.
I'll just say that there were a number of "miraculous" or unexplained aspects associated with aliens that are briefly contacted. These seemed questionable to me - a person inclined to skeptical about such things. These aspects only take place in a small section of the book. If anyone is really curious about what aspects I mean, let me know and I'll elaborate.