This is the story of the biggest surveillance operation in British history, and how British intelligence agencies and their allies thwarted the deadliest terror plot ever devised on home soil. In June 2006, Abdullah Ahmed Ali flew back from Islamabad, Pakistan to return to his east London home. Ali had no idea he was being watched by the British security service, MI5. The airport discovery marked a critical moment, as they found a collection of batteries and a large bag of the soft drink powder. MI5 then launched their largest surveillance operation, Operation Overt, with a breakthrough arriving when Ali was seen visiting an empty flat recently bought in Walthamstow. MI5 broke in and planted surveillance equipment in the flat. A camera recorded Ali and his friend, drilling a hole in the bottom of a soft drinks bottle, to enable them to empty the bottle and replace its contents with liquid explosives. Being tracked by MI5, Ali met another of his co-conspirators in a car park beside Waltham Forest Town Hall, where he handed over several suicide videos. The police immediately moved in to arrest them and held a total of 24 suspects in a series of co-ordinated raids. 

This is the story of Robert Hanssen - a former FBI agent who systematically dismantled the United States' network of undercover Soviet agents, from the inside out. His espionage was described by the Department of Justice as "possibly the worst intelligence disaster in U.S. history". Hanssen spied for Soviet and Russian intelligence services against the United States from 1979 to 2001 and his actions pitted CIA against FBI and triggered a mole hunt that brought the American intelligence services to their knees.