The title suggests that Professor Amos Shute’s intrepid travelers are going back in time to
four planets that are identical in every way to our own, but then again, perhaps those four
planets were merely at earlier times to begin with. We won’t say one way or another, but we
are glad that the Spanish Flu pandemic, World War I, World War II, and World War III were all
averted on some Earth.
— Michael Main