1999, aimen dean embarks on a mission that might be his last -- returning to the al qaeda camps in afghanistan, no longer a jihadi but now a spy for british intelligence. >> i was talking a bus with one of my friends and i remember thinking about i'm going to meet my comrades but this time not as a friend but i'm going to see them as targets. >> reporter: his ability to live among former comrades and report on their activities would be rare and invaluable. >> you cannot replace human intelligence and the ability to get next to somebody when they are suspicious that you're trying to spy on them and they'll only talk to somebody they view as a friend. you can't replace it. >> reporter: and british intelligence would not be able to replace a secret agent like dean if he were caught.