As a little girl in Callander, Scotland, in the earliest days of the 20th century, Helen MacFarlane was known for acting like a tomboy. Her rowdiness and sometimes-violent temper gave rise to her nickname, "Hellish Nell.” But she also known for something else—her seeming ability to communicate with spirits and her frequent visits from ghosts.

Banished from the family home at age 16 after getting pregnant, she went on to marry a devoted Spiritualist named Harry Duncan, who believed in her powers. In the wake of World War I and its massive death toll, communicating with the dead via spirit mediums became a popular pastime, and the freshly named Helen Duncan found a new mission in life: she became a Spiritualist medium. (Though the term “Spiritualist” is often misused today to mean someone who is “spiritual,” it was once a flourishing religion that involved communicating with the dead.)

Duncan earned her living traveling throughout Britain, conducting séances at spiritualist societies and in private homes and charging admission for her services. Duncan was known as a "materialization medium"—someone who could not just commune with the dead but produce physical manifestations of them. Her séances frequently included strings of otherworldly white ectoplasm produced from various orifices, as well as ghostly images of the faces and bodies of departed "spirit guides."

However, a 1931 investigation by famed psychic researcher Harry Price concluded that the ectoplasm was actually cheesecloth covered in egg whites, iron salts, and other chemicals, which Duncan stored in her stomach and then regurgitated. The "spirits" were pictures cut from magazines, while a “spiritual hand” glimpsed in one séance was revealed to be a rubber glove. Price's investigations failed to dim enthusiasm for Duncan's séances, however. Neither did a 1933 trial and imprisonment for fraudulent mediumship, which resulted after one of Duncan's spirit guides, "Peggy," was revealed to be a vest. As the cultural historian Malcolm Gaskill wrote for History Today, “Spiritualists … thrived on feelings of persecution by orthodox science, organized religion, and, above all, the police, who sought to protect the public against imposture. Accordingly, Helen Duncan was lionised and her fame grew to the extent that even a conviction for fraud at Edinburgh in 1933 saw her hailed as a martyr."

After the outbreak of World War II, Duncan's services were especially in demand. The spirits offered consolation amid fear and despair, and in some cases, even shared information that seemingly broke through the tight shroud of secrecy the government had imposed. But it was this wartime climate that proved to be Duncan's undoing.

In November 1941, the battleship HMS Barham was sunk by German torpedoes, with more than 800 lives lost. The British government censored news of the sinking to protect morale; by some reports, they even forged Christmas cards from dead sailors to their families. A few months later, however, at a séance in Portsmouth (the town where Duncan lived, which also happened to be home to the Royal Navy), Duncan told a mother that her son had appeared wearing a hatband with the words HMS Barham on it and saying: "My ship is sunk."

When news of the séance reached officials, they were appalled. And once preparations for D-Day began, they decided to take action. By some accounts, Duncan had also revealed specific details of the sinking of the HMS Broadwater in 1941, and there were concerns that her information—whatever its source—would endanger the secrecy needed for a successful invasion of occupied France.

In January 1944, police burst into one of Duncan's séances, arresting her and three members of the audience. She was originally charged under Section 4 of the 1824 Vagrancy Act, which was commonly used at the time to punish offenses related to fortune-telling, astrology, and spiritualism. Such charges usually resulted in no more than a fine. But Duncan's case was different: as Gaskill notes, "at this most sensitive point in the war the authorities wanted her in prison." In March, Duncan was prosecuted at London's Old Bailey for conspiracy to contravene the Witchcraft Act of 1735, the first change of its kind in more than a century.

Despite what it sounds like, the Witchcraft Act wasn't meant to prosecute actual witches, so much as punish people for pretending to have the powers of a witch. During the trial, which was a media sensation, Duncan was accused of pretending “to exercise or use human conjuration” so “spirits of deceased persons should appear to be present.”

Her lawyer, a spiritualist himself, attempted to defend her by proving she wasn’t just pretending. He called more than 40 witnesses who had seen Duncan's powers at work, and even offered a private séance to the jury (they declined). The defense, however, was unsuccessful, and Duncan was imprisoned for nine months at North London's Holloway women's prison, the last person to be jailed under the act.

Winston Churchill, who was then prime minister, denounced Duncan's conviction as "obsolete tomfoolery." By some accounts, he also visited her in jail. In 1951, he finally repealed the 200-year-old Witchcraft Act, but Duncan’s conviction stood. She died five years later, shortly after yet another police raid. To this day, family members and others are working to clear her name.