The team reinvestigates the 1950s murder of a young wages clerk at Battersea Power Station.
Richard Dunne was hanged for Frederick Tully's murder, but his granddaughter, Hannah, is waging a campaign to have Dunne posthumously pardoned.
The discovery of a suitcase containing used fivers in the attic of the Tully home sheds new light on the case, revealing a possible blackmail plot.
A trace on the used fivers reveals that they should have been destroyed in the Battersea Power Station furnaces back in the 1950s, but were actually being saved by Douglas Murray, a friend of Tully's.
It also becomes clear that the Battersea furnaces were being used to destroy other sensitive documents as well as used bank notes.
When the team discovers a classified document relating to a massacre of civilians carried out by the British Army in Kenya, they uncover a blackmail plot spanning more than 50 years.