It was the most devastating terrorist attack the world has ever seen. Since September 11th Panorama has followed the trail the hijackers left across three continents. Jane Corbin, the reporter who has tracked Osama Bin Laden's terror operations for many years, investigates what drove them on a mission to kill thousands of people and themselves. The documentary reveals the planning, the tradecraft and the financing of a secret terror cell in Europe and why the cell went undetected. This is the inside story of 19 men whose terrorist atrocities changed the world forever.