It's been 20 years since the event that devastated the planet.

Cat and Simon, children of the apocalypse, are living happily in what was once Colorado. But then, an ominous reminder of the dark world of their parents' past shatters their peace. Something has changed.

They journey back to their parents' community in Yellowstone for answers ... only to find it empty!

Where is everybody? What is happening?

Is this the beginning of the end?