(Series retrospective below--scroll down past the cut)
Sigh. I really shouldn't have gotten my hopes up after #13. Begin Again isn't badly written; in fact, the structure and prose are a noticeable improvement over #12, the last time we joined the Remnants and Marauders on Earth. However, this book totally fails to deliver on any of the interesting hints set up in previous installments, and the way everything is solved at the end is forehead-slappingly nonsensical. Let me break it down:
The plot of this book centers around something called the regreening ritual (get used to hearing that term a lot), a process whose mechanisms are never described but which is supposed to transform Earth from a barren wasteland to a green living planet again. After Tate crashed Mother on Earth centuries before the Rock hit, Billy reunites with the ship. We learn that three elements are needed for the regreening--the ship, the five embodied in Billy (aka the Missing Five) and a third unspecified element, which--are you ready for this?--turns out to be Echo's baby.
Are you following all this? No? Good. Because neither am I. None of this makes any sense. Not only are we expected to believe a derelict ship, a boy possessed by five ghosts (what else are they? Tell me), and a baby can speed-terraform the Earth, we're expected to believe none of the other Remnants question how this is possible. I can understand maybe Mo'Steel having faith, as he says he's Catholic, but I felt at least Jobs and other scientifically minded characters should have paused to say, "Huh?" These people grew up in the 21st century for God's sake; they should have at least a baseline understanding of chemistry and physics and biology and know Things Don't Work Like That. Instead, it's actually the Marauders who question what's going to happen and what the consequences for the world will be. Because they're so recalcitrant and primitive, y'all *eyeroll*
Yet they ask sensible questions that the book never bothers to answer. In the end, we aren't even shown the actual regreening ritual--I suspect because the writer(s) knew they were pulling things out of their hat like a magician in the last five minutes of his act. Don't get me started on the epilogue, where everyone's shown having shacked up like Brady Bunch happy families in an Edenic paradise and oh, there happen to be Earth animals again because reasons.
And no, we never find out what Billy is.
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Series retrospective: Remnants was a reread for me. I read it as a teen when it first came out in 2002-2003, and remembered being vaguely bemused at the storyline. I don't think I was ever invested enough to feel cheated at the end, which says something.
On the reread, what struck me most was missed potential. 80 people escape the wholesale destruction of Earth only to wake up 500 years later on an alien starship, with weird mutations and hostile beings jockeying for power? This could have been great. It could have been a YA Battlestar Galactica meets Voyager. And I think for much of its run, Remnants at least makes a good faith effort to deliver on its premise: it has weird and awesome set pieces, strange powers, tantalizing hints at a bigger metaplot scattered throughout.
But at about the midway point, something happened. Much like Mother's course, the narrative was diverted back to Earth for the thinnest of excuses, and everything started to unravel after that. Being honest with you, I would have been so onboard if, once they gained control of Mother, the Remnants had started scouring the universe--much like Tate does--for habitable planets elsewhere. Only after realizing the universe is mostly empty would they return to Earth in desperation and attempt to use Mother's world-creation engines to regreen the Earth, knowing it would take decades if not centuries, but determined to restore the one world they know for certain could support life. The world that was their home, and could be again.
I know I just basically pulled a How It Should Have Ended on Remnants, but the fact I could come up with a better endgame plot in literally 10 minutes is a testament to how broken this story becomes after the halfway point. This could have been great, but it wasn't, and it's time to move on.
TL;DR - Go read Animorphs instead.