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.