It is the middle of the 21st century

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 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 a threat to all the worlds of the 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 . . .