Human society continues to evolve on Datum Earth, its battered and weary origin planet, as the spread of humanity progresses throughout the many Earths beyond. Lobsang, now an elderly and complex AI, suffers a breakdown and, disguised as a human, attempts to live a "normal" life on one of the millions of Long Earth worlds. His old friend Joshua, now in his fifties, searches for his father and discovers a heretofore unknown family history.

"After the cataclysmic upheavals of Step Day and the Yellowstone eruption, humanity is spreading further into the Long Earth Society, on a battered Datum Earth and beyond, continues to evolve. And new challenges emerge. Now an elderly and catankerous AI, Lobsang is living with Agnes in an exotic, far-distant world. He's determined to lead a normal life in New Springfield--they even adopt a child. But there are rumors, strange sightings in the sky. On this world, something isn't right ... Millions of steps away--learning about a hidden family history and the father he never knew--Joshua receives an urgent summons from New Springfield. Lobsang has come to understand that what has blighted his Earth is a threat to all the worlds of Long Earth. To counter this threat will require the combined efforts of humankind, machine, and the superintelligent Next. And some must make the ultimate sacrifice ..."--

Lobsang's pursuit of a human life, Joshua's search for his father and the adaptation efforts of "the Next" post-humans are challenged by a voracious alien race that would conquer and colonize the Long Earth.

Human society continues to evolve on Datum Earth, its battered and weary origin planet, as the spread of humanity progresses throughout the many Earths beyond. Lobsang, now an elderly and complex AI, suffers a breakdown and, disguised as a human, attempts to live a "normal" life on one of the millions of Long Earth worlds. His old friend Joshua, now in his fifties, searches for his father and discovers a heretofore unknown family history.