The final episode opens with the new tenant in John Christie’s former flat at 10 Rillington Place uncovering a hidden alcove and making a grisly discovery.

The story then flashes back three years, when Christie was the main prosecution witness in the murder trial of his former neighbour Tim Evans.

As Christie calmly and convincingly lies under oath, you can sense Evans feeling the noose tightening around his neck.

Christie’s wife Ethel backs up her husband’s story in court and, in doing so, helps send the wrong man to the gallows.

It is an action for which she will pay dearly. And as the tale is told from Christie’s point of view, we see for the first time, the killer at work in chilling, clinical detail.