It is the middle of the twenty-first century. After the cataclysmic upheavals of Step Day and the Yellowstone eruption, humanity is spreading farther into the Long Earth. Society, on a battered Datum Earth and beyond, continues to evolve. And new challenges emerge. Now an elderly and cantankerous 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 also a threat to all the worlds of the Long Earth. Countering this threat will require the combined efforts of humankind, machine, and the super-intelligent Next. And some must make the ultimate sacrifice. . . .