In this case, we're still following Joshua Valiente as he investigates those who are evolving out of the Long Earth, Sally Linsay as she and her father cross "The Gap" to Mars, and Maggie Kauffman as she leads a crew out to Earth West 250 million. Lopsang is less of a presence in this entry, although he does have an overarching narration to try to tie together the disparate plot threads.
Joshua's narrative involves what may or may not be a new speciation of humans, the youth who call themselves "The Next". We encounter them in both Joshua's tale and in Maggie's journey when she fings the wreck of the
Maggie's voyage focuses on the variety of of life as it evolved elsewhere in the infinite Earths, and provides tales of human compassion missing elsewhere. Maggie is kind of the conscience of humanity in this. She takes a Beagle (sapient bipedal dogs introduced in the last book) onto her crew along with several trolls. This leads to revelations on why the Beagles really dislike some of the human scientists in their world. She tries to find a way to get medical help for a creature someone shoots at (through a window) in the really high numbered Earths. And she's the one who ultimately gets to decide the fate of the Next after running across a group of them from Happy Landings way out in the 200 millions.
Sally, who's not exactly happy with her father, none the less accompanies him to Mars. "The Gap" (the first iteration of Earth going West where there is no Earth [likely caused by an asteroid collision early on]) has a Mars with vegetation. (The idea that gets relayed here is that as life crossed the gap, DNA managed to hit Mars. There's also the idea that variations on Mars had the right volcanic melting of ice that allowed life to flourish for however long.) Their journey finds that there is life on "Joker Marses", where the conditions for life are still being met or have been met previously. And ultimately, their journey os one of trying to figure out if the ends justify the means. Yes, "Step Day", unleashed by Willis, wound up being a net positive, but the cost of life was very high. The sacrifices he makes on Mars looking for something specific may also have similar benefits for humanity, but there's an equally large sacrifice made in the process. The comparison to Greek mythology's Daedalus is made about Willis Linsay, which seems rather fitting in the end.
It's not a terribly long book, but it's fun to read. And it does seem to capture the essence of humanity in all of its narratives, capturing moments of fear, hatred, hope, and joy.