The cataclysmic Yellowstone eruption is shutting down civilization. Populations flee to the safety of stepwise Earths. Sally Linsay, Joshua Valiente, and Lobsang are all involved in the perilous clean-up.

But Joshua faces a crisis close to home. From deep in the Long Earth, a new breed of super-bright post-humans is emerging—but "normal" human society is turning against them.

Meanwhile, Commander Maggie Kauffman embarks on an expedition to the unexplored limits of the Long Earth, and Sally is contacted by her father, Willis—inventor of the original Stepper. He is planning a voyage of his own—across the Long Mars. But what is his true motivation?

For Joshua, for humankind, for the Long Earth itself—everything is different now.