With Sandra Rivett lying dead in the basement, Lucan must decide whether to face the police or run.

And so begins the second mystery that has made this case so compelling.

Where did Lucan go that night?

Was he being sheltered by his friends who the police nicknamed The Eaton Square Mafia?

Alex von Tunzelmann pieces together what we know of the hours after the murder, asking whose version should we believe.

She meets an eyewitness who says she was the last person to see Lucan alive, and crawls underground into a bunker where the police were sure he was hiding.

Producer: Sarah Bowen