In the early hours of the morning on 23 March 1857, warehouse clerk Emile L'Angelier staggered through the dark Glasgow streets towards his lodging house, doubled over in pain. By the next morning he was dead of arsenic poisoning. Within a few hours stacks of illicit love letters were discovered suggesting that the young man had been involved in a highly passionate secret affair - and Madeleine Smith, the upper-class daughter of a wealthy Glasgow businessman, was arrested for Emile's murder.