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 to accompany him. But Sally soon learns that Willis has an ulterior motive for his request… Meanwhile U. S. Navy Commander Maggie Kauffman has embarked on an incredible journey of her own, leading an expedition to the outer limits of the far Long Earth. For Joshua, the crisis he faces is much closer to home. He becomes embroiled in the plight of the Next: the Long Mars

2045-2059. After the cataclysmic upheavals of Step Day and the Yellowstone eruption humanity is spreading further into the Long Earth, and society, on a battered Datum Earth and beyond, continues to evolve. Now an elderly and cantankerous AI, Lobsang lives in disguise with Agnes in an exotic, far-distant world. He’s convinced they’re leading a normal life in New Springfield – they even adopt a child – but it seems they have been guided there for a reason. As rumours of strange sightings and hauntings proliferate, it becomes clear that something is very awry with this particular world. Millions of steps away, Joshua is on a personal journey of discovery: learning about the father he never knew and a secret family history. But then he receives a summons from New Springfield. Lobsang now understands the enormity of what’s taking place beneath the surface of his earth – a threat to all the worlds of the Long Earth. To counter this threat will require the combined efforts of humankind, machine and the super-intelligent Next. And some must make the ultimate sacrifice