Earth has been conquered by an alien race known as the Assembly. The human adult population is gone, having succumbed to the Tone-a powerful, telepathic super-signal broadcast across the planet that reduces them to a state of complete subservience. But the Tone only affects the population once they reach their early twenties. Which means that there is one group left to resist: Children. In Valley of Fires the trio of kid heroes - Holt, Mira, and Zoey - are forced apart to accomplish individual quests if they are to have any hope of uniting what remains of Earth's disparate survivors against its alien invaders. Mira ventures west to bargain with Van Cleef, the enigmatic leader of North America's most infamous resistance group. But Van Cleef has his own plans to end the conflict, a destructive solution that might actually work...but at a horrible cost. Meanwhile, Holt travels with Ravan and Avril back to the one place he swore he would never return: Faust, the sprawling and dangerous desert city of the Menagerie pirate guild. He goes not only to resolve his issues with Tiberius, its tyrannical leader, but to enlist the Menagerie in the fight to save Zoey. Except Tiberius has his own problems. Factions within the Menagerie are splintering, and word of rebellion is beginning to spread. If Holt wants the Menagerie's help, he might have to help his greatest enemy in exchange. Traveling separate paths with little hope of safety or reunion, Holt and Mira bring this thrilling, genre-bending series to an utterly unforgettable close.

Earth has been conquered by an alien race known as the Assembly. The human adult population is gone, having succumbed to the Tone-a powerful, telepathic super-signal broadcast across the planet that reduces them to a state of complete subservience. But the Tone has one critical flaw. It only affects the population once they reach their early twenties. Which means that there is one group left to resist: Children. Holt Hawkins is a bounty hunter, and his current target is Mira Toombs, an infamous treasure seeker with a price on her head. It's not long before Holt bags his prey, but their instant connection isn't something he bargained for. Neither is the Assembly ship that crash-lands near them shortly after. Venturing inside, Holt finds a young girl who remembers nothing except her name: Zoey. As the three make their way to the cavernous metropolis of Midnight City, they encounter young freedom fighters, mutants, otherworldly artifacts, pirates, feuding alien armies, and the amazing powers that Zoey is beginning to exhibit. Powers that suggest she, as impossible as it seems, may just be the key to stopping the Assembly once and for all.