4500 planes are in the air over North America on the morning of September 11, 2001. After four horrifying crashes, no one knows if any other aircraft might be highjacked by terrorists, about to commit further acts of destruction. Where can 4500 planes, and thousands of passengers, safely land? And which might be Al-Qaeda missiles? At that very moment, 500 planes are westbound toward the US. 233 turn back to where they came.