The stepper enables a person using it to step sideways into another America, another wherever that person happened to be, another Earth. And if the person using it keeps on stepping, they keep on entering even more Earths. This is the Long Earth. And the further away a stepper travels, the stranger -- and sometimes more dangerous -- the Earths become.

"2040-2045: In the years after the cataclysmic Yellowstone eruption there is massive economic dislocation as populations flee Datum Earth to myriad Long Earth worlds. Sally, Joshua, and Lobsang are all involved in this perilous rescue work when, out of the blue, Sally is contacted by her long-vanished father and inventor of the original Stepper device, Willis Linsay. He tells her he is planning a fantastic voyage across the Long Mars and wants her...

The Next--the hyper-intelligent post-humans--realize that the missive they received from the center of the galaxy contains instructions for kick-starting the development of an immense artificial intelligence knows as The Machine. But to build this computer the size of an Earth continent, they must obtain help from the more populous and still industrious worlds of mankind.