Radio 1 Presenter Sian Eleri investigates the story of Scottish medium Helen Duncan, who in 1944 became the last person convicted and imprisoned under the Witchcraft Act. As the Blitz rained down on London, Helen went on trail at the Old Bailey in a case that would make headlines around the world. Revered by her supporters, who believed she had the power to bring back spirits of the dead, Helen became the most notorious medium in history, and people flocked to have seances with her. Sian begins by tracking down former criminal lawyer Graham Hewitt, who is fighting to pardon Helen on behalf of her surviving family members. Sian discovers it was actually fraud for which Helen was put in prison using witchcraft law, but Graham highlights what he believes is inexplicable evidence from her trial that proves otherwise.

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There is an unnerving breakthrough in Sian’s investigation into a woman who claimed to summon spirits of the dead when she finds an audio recording of a Helen Duncan seance. Not only is a gravelly male 'spirit' heard to speak, but the people gathered comment that they see and recognise him. How is this possible? Sian tests out different theories – was it someone else speaking? Helen putting on voices? Did she have accomplices? How does that explain someone recognising a dead relative? Searching for answers and hoping to understand how Helen was convincing people they were communicating with their dead loved ones, Sian tracks down a modern-day medium. He claims to commune with the other side, but a seance in Edinburgh leaves Sian with more questions than she first started with. And Helen’s claims to be able to manifest the physical dead also take things to another level to what she has just experienced.

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There’s only a handful of mediums worldwide who claim to do anything close to what Helen Duncan said she could and physically manifest spirits into a room. For Sian, the only way to fully understand what was happening at Helen’s séances is to witness this for herself, so she tracks one of these rare mediums down in the Netherlands. Sian arrives at the Dutch monastery and is greeted by a host who takes Sian on a private tour of the séance room, where the medium will work that evening. The traditions haven’t changed much since Helen’s 1930s séances. The medium will be searched, then tied to a chair, enclosed behind a black curtain, and spirits, depending on what unfolds that night, may physically 'appear'. The room will be pitch black, and Sian will be locked inside.

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Sian’s mission to get to the truth behind Helen Duncan’s apparently supernatural abilities draws to a close – but a secret package sent from Helen’s granddaughter in America leaves Sian with what appears to be the smoking gun she has been waiting for. The package contains what seems to be unexplainable proof that a 'spirit' revealed the sinking of a second warship during one of Helen’s séances. But this time, the séance happened before even the government was aware. Sian tussles with the few explanations still on the table – was this information received from beyond the grave? Or could Helen Duncan have been a spy and in league with the enemy? Wherever the truth lies, Sian feels increasingly sure there was more to Helen’s Old Bailey trial and conviction than a case of fraud.

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