21 December 1988. A bomb detonates on board Pan Am flight 103 above Lockerbie. The attack kills 270 people. The Scottish police and FBI must join forces to find the perpetrators.

In the weeks following the bombing, the pressure to make progress is immense. The joint Scottish-FBI investigation closes in on its first suspect.

The Scottish police head to the US with new evidence and a potential breakthrough. But the CIA appears to be withholding crucial intelligence on a possible bomb-maker.

The Frankfurt luggage list and bomb-damaged clothing leads Scottish CID to a small clothes shop on the island of Malta. Was this the origin of the bomb?

While the joint Scottish and FBI investigation closes in on the identity of the 'mystery shopper', a CIA asset could hold the key to the identity of the Pan Am 103 bombers.