Promises Stronger Than Darkness marks the final installment of the international bestselling author Charlie Jane Anders's absolutely heart-stopping YA series, Unstoppable. They're the galaxy's most wanted—and our only hope.

When Elza became a space princess, she thought she'd be spending her time at the palace, wearing gorgeous couture and soaking up everything there is to know—but instead, she's on the run, with everyone hunting for her and her friends.

Rachael followed her best friend Tina on the adventure of a lifetime—but now Tina's gone, and Rachael's the only one keeping her friends together, as they go on a desperate quest to save everyone from an ancient curse.

Rachael, Elza, and their friends have found one clue, one shining mysterious chance to stop the end of the world. And that takes them back to the second-to-last place they'd want to be: enlisting the aid of Captain Thaoh Argentian, the woman who stole Tina's body (and who now seems to be relishing a second chance at teenage chaos and drama, instead of living up to her legacy as an intrepid heroic commander).

With only a ragtag band of misfits, crewmates, earthlings, friends, lovers (and one annoying frenemy), the Unstoppable Crew are up against the universe—and they soon find that in order to survive, they may have to cross a line they vowed never to cross.