There will be some spoilers for the ending to Reboot.
At the end of Reboot, Wren successfully broke into the Austin compound to save Callum and also free every single Reboot. She even managed to get the serum to cure him of the disease that HARC (The bad government guys) gave him, the one that made him want to eat everyone. In the final scene, Wren, Callum, and a bunch of other Reboots arrive at the entrance to the Reboot territory.
They are welcomed by Micah, the leader. He is immediately impressed by Wren, who is the only one with a number higher than his (Micah is at 160 or so). As they spend more time there, it becomes clear that being at the Reboot camp isn't much different from being in the compound. They are still seen as a number rather than a person.
The last straw is when Micah comes up with a plan for the Reboots to overthrow Austin. Because Wren and Addie freed all the Reboots, the city has very little HARC presence left. It's now a simple matter of marching in and killing every human on sight. Yeah, Micah is pretty awful. He believes that the humans would attack Reboots eventually, so he may as well attack first.
Wren has never spared any love for humans. She doesn't want to destroy them, but she also doesn't want to fight for them. Callum still has a human family, plus he feels closer to humans because of his lower number. He actually grows into his own in this book, coming up with plans to overthrow Micah and help the humans in Austin get rid of HARC.
I really wish that I had taken the time to re-read Reboot before reading the sequel, because it was hard to remember a lot of little details. I do enjoy that a lot of books are sticking with duologies instead of trilogies. Rebel was a great conclusion to Reboot. I liked the first book better, but mostly because of all the sweet first love stuff with Wren and Callum. Reboot was also action-packed and had a satisfying ending.
I received my copy of Rebel from Edelweiss, courtesy of HarperTeen. It's available for purchase now.