The team reinvestigates the Fifties murder of Frederick Tully, a young wages clerk at Battersea Power Station, as the drama about an unlikely team of crack detectives continues. Patrick Dunne was hanged for 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. And when Sir Edward Chambers, the key witness against Dunne, retracts his evidence, the team has reason to question the original investigation. However, with Hannah reluctant to co-operate with the police, it is left to Gerry Standing to charm her mother, June, in order to gain an insight into the case, which Standing takes a little too literally.

The original case against Dunne is weakened when the team discovers that the man leading the investigation was Tommy Collinson, an officer known for bending the rules and falsifying evidence in order to gain a result. Sir Edward found the murder weapon on a bus, but, when the team visits him to go through his statement, he claims that Collinson misled him into wrongly identifying Dunne as the man he saw that night.

A trace on the used fivers reveals that they should have been destroyed in the Battersea Power Station furnaces back in the Fifties, 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, the most useful of which were once again rescued by Murray and Tully. 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.

Bank Holiday Monday 7 May
9.00-10.00pm BBC ONE