This is really half a book, so it is hard to review. Not the first part in a duology, but truly half a book. There is no plot convergence. Nothing gets solved. There are characters that have a POV for a few chapters and then disappear, without us knowing why we get to look at their lives (Mary Kent, Douglas and Ernest). Are they the same people as Merope, Polly and Michael on different trips, or are they not? Let's hope All Clear will bring this all together.
That said, it's Connie Willis in her best alternate universe: the time traveling Oxford historians of the late 21st century, and it's fascinating as usual. However, if you want to start reading Connie, you should start with either Doomsday Book (great, but as the title foreshadows, a bit depressing) or the very excellent To Say Nothing of the Dog. That
one I reread every couple of years.