Lobsang, now an elderly and somewhat over-complex AI, seems to have suffered a breakdown of sorts. With Agnes, he now lives, in disguise, in an exotic High-Meggers world -- 'New Springfield' --- determined to lead a 'normal' life as a human colonist. They even adopt a child, a son. But it seems that Sally Linsay guided them there for a reason.
For it's at New Springfield that the Long Earth chain of worlds has become 'entangled' with another Long world -- an alien planet. Strange haunted-house sightings in a cellar reveal this alien world to be inhabited by voracious colonising cyborgs --- 'silver beetles' -- who seem to be modifying this world for their own terrifying purposes.
Elsewhere Joshua, now in his fifties, tries to track down the father he's never known. In so doing, he unravels a deeper history -- of natural steppers who banded together in Victorian London. At first they offered their services to the crown, but were eventually hunted down and their secret covered up. But their story didn't end there, and it would seem that an ancestor of Joshua's became embroiled in a clandestine 'Darwinian' programme involving these 19th century steppers.
Free from human control, the super-intelligent Next are growing up. As they debate how to 'manage' the vast herd of humans they live amongst, one of the brightest of them all begins to preach about the unity of all human kinds and in so doing stirs up both human and Next communities -- but then, still only young, he faces a new duty.
Meanwhile, on New Springfield, Lobsang has been joined by Joshua and they've discovered the ominous ultimate goal of the cyborg silver beetles and the vast machines they have created. The threat they pose to the Long Earth -- and other Long worlds -- convinces Lobsang and the others that the beetles' world has to be 'cauterised', taken out of the Long Earth chain, to prevent the spread of their kind. To achieve this will require super-stepping abilities -- and for some it will mean making the ultimate sacrifice.