A dense black cloud boiled up in the southeastern sky. It rose high and fast, like a time-lapse movie of the birth of a thunderhead. But it was no rain cloud. Wholly black, it reached up and up until it loomed over her, blocking out the sun. Somehow, she knew, it was Death coming for her.

Pre-med student Coral is on vacation in Idaho when something terrible happens. The black cloud is followed by a wildfire and searing heat that lasts for days. She survives deep in a cave but emerges days later to find the world transformed, with blackened trees, an ash-filled sky, and no living creatures stirring - except for her.

So begins her desperate journey to find water and food and other survivors...and the answer to the mystery of what happened.