"Ashfall" is a novel by Mike Mullin that follows the story of a young boy named Alex as he navigates a post-apocalyptic world after the supervolcano Yellowstone erupts. The eruption causes widespread destruction, including ash fallout that disrupts life and creates dangerous conditions. Alex must find his way to safety while facing various challenges and meeting other survivors. The book explores themes of survival, resilience, and the impact of natural disasters on society. As Alex journeys through the ash-covered landscape, he learns valuable lessons about friendship, courage, and the importance of hope in dire situations.