The lovers’ bodies were found in a cellar.

Thomas Kinnear, the owner of the home, had been shot in the left side of his chest. Nancy Montgomery, his housekeeper and paramour, was struck in the head with an axe and then strangled. Her body was discovered crammed beneath a tub. An autopsy would later reveal that Montgomery had been pregnant when her life came to an abrupt end.

It was July of 1843 in Upper Canada, a British colony located within what is now the province of Ontario. Kinnear, a gentleman of Scottish origin, owned property in a rural village some 16 miles outside of Toronto. Conspicuously absent from his house in the wake of the murders were his two domestic servants: 20-year-old James McDermott and 16-year-old Grace Marks. Both were Irish immigrants who had started working for Kinnear just weeks earlier. McDermott had previously served as a soldier in a Canadian regiment, while Marks had worked as a servant in a number of different households. The pair appeared to have fled Kinnear’s home with a hoard of stolen goods.

From the get go, investigators suspected that McDermott and Marks had been involved in the grisly crime. But whether both parties were equally culpable proved to be a more elusive question—one that remains shrouded in mystery to the present day.

Not long after the murders, McDermott and Marks were tracked down in Lewiston, New York and arrested. At their trial in Toronto, McDermott was convicted of first-degree murder and Marks as an accessory before and after the fact in the case of Kinnear. Both defendants were sentenced to death for their crimes, and it was deemed redundant to try them for Montgomery’s murder as well. McDermott was promptly hanged. But in Marks’ case, the jury recommended mercy—possibly because she was so young—and officials commuted her sentence to life imprisonment.

More than a century later, Marks’s story captured the attention of Canadian author Margaret Atwood. In the 1960s, before she became a well-known writer, Atwood read about Marks in the book Life in the Clearings Versus the Bush, a chronicle of 19th-century pioneer life by Susanna Moodie, an English emigrant to Canada.

Atwood would mull over the Kinnear-Montgomery murders for decades, writing a number of acclaimed novels —including The Handmaid’s Tale—in the meantime. Finally, in 1996, she published Alias Grace, a novel that blends the events of the double homicide with flourishes of liberal invention to reconstruct the circumstances surrounding the crime. The book is set more than ten years after Marks’ conviction and casts her as a somewhat impenetrable narrator, who tells her version of events to a psychiatrist interested in her case. On November 3, Netflix, in conjunction with the Canadian Broadcasting Corporation, will release a miniseries adaptation that delves into many of the same questions as its source material: What happened on the day of the killings? What role did Marks play in them? And when history is reflected through a prism of preconceptions and prejudices, can the truth ever be known?

The trial of Marks and McDermott caused a sensation in 19th-century Canada. The press gleefully covered the story, which sizzled with intrigue, gore and hints of illicit sexuality. The slain lovers, after all, were not married and belonged to opposite ends of the class hierarchy. On the day of McDermott’s trial, so many spectators packed into the courtroom that “some alarm was created by a report that the floor of the courtroom was giving away,” according to a summary of the trial proceedings that appeared in a special edition published by the Star and Transcript newspaper.

Marks, however, was a source of particular intrigue. She displayed little emotion during the trial proceedings, though is said to have fainted when her sentence was read. Bizarrely, according to newspaper reports, she showed up to court wearing clothes that she had stolen from the dead Nancy Montgomery. And as the Examiner newspaper observed at the time that there had been “considerable interest in the trial,” due in part to “some doubt whether the female prisoner had been a willing or reluctant participant in the murder.”

Even though the case was widely reported on, few hard facts emerged. Atwood once noted that in her research, she found that “the witnesses—even the eye-witnesses, even at the trial itself—could not agree” on what they had seen. The defendants, Marks and McDermott, gave multiple, incompatible accounts of the crime, though neither claimed to be completely innocent of it.

In Marks’ last confession, published in the Star and Transcript booklet, Marks said that after Montgomery had fired McDermott “for not doing his work properly,” he decided to kill her and Kinnear. “[H]e had made me promise to assist him,” she said, “and I agreed to do so.” Marks claimed that she tried to run away from the house after Kinnear was killed, prompting McDermott to shoot at her. Witnesses testified to finding a ball from the weapon lodged in a door near the kitchen.

McDermott, on the other hand, flipped the narrative in his testimony, insisting that Marks had goaded him until he agreed to help her commit the murders. And she had been fired by Montgomery, he claimed. “She said she had been warned to leave, and she supposed she should not get her wages,” McDermott testified. “She said … ‘I'll assist you, and you are a coward if you don't do it.’ I frequently refused to do as she wished, and she said I should never have an hour's luck if I did not do as she wished me.”

On the day he went to the gallows, McDermott added a statement to his confession. Marks, he said, followed him into the cellar after he had struck Montgomery with an axe, wounding but not killing her. Marks “brought a piece of white cloth with her,” the statement reads, “tied the cloth tight round [Montgomery’s] neck and strangled her.”

In the afterword to Alias Grace, Atwood notes that she “felt free to invent” details to fill in the gaps between irreconcilable versions of the murders. For modern-day researchers, who cannot take such liberties, it is impossible to figure out what exactly happened at the Kinnear homestead. But the case is nevertheless intriguing because it exemplifies “conflicting notions” of female killers in the 19th century, says Kathleen Kendall, an associate professor of sociology at the University of Southampton.

Marks proves so fascinating, Kendall theorized in an interview with Smithsonian.com, because the murder charge flouted Victorian-era conceptions of femininity, which deemed women gentler and more “morally pure” than their male counterparts.

Lizzie Seal, author of Women, Murder and Femininity: Gender Representations of Women Who Kill, agrees. “Women are seen as being masculine, if they've committed violent crimes,” she says. “In the 19th century, that portrayal did emerge, in relation to servants particularly ... As working class women who were doing very manual labor, heavy labor as part of their duties, they did not meet a Victorian lady type of ideal.”

On top of that, Marks’ status as a domestic servant made her a doubly unnerving figure. Contemporary newspapers, which were largely published and read by a demographic that depended on servants, seized on the unthinkable subversion perpetrated by Marks and McDermott, who appeared to have killed their employer without much in the way of provocation. “A very dangerous neglect as to the requiring of ‘characters’ with servants prevails among us,” the Examiner wrote while covering the trials in November of 1843. As a female servant involved in the murders, Marks may have come across as an especially anomalous character.

But not all commenters cast Marks as the gender-subverting instigator of the crime. Other accounts emphasized her youth, her beauty, or her purported pliability to suggest that she was an unfortunate and vaguely stupid girl who had fallen victim to an overweening male villain. The Star and Transcript court summary, for instance, described McDermott as having “a swarthy complexion, and a sullen, downcast, and forbidding countenance.” Its portrayal of Marks was somewhat more generous. She was “rather good-looking than otherwise,” the paper opined, and appeared “totally uneducated”—incapable, perhaps, of masterminding a double homicide.

Deeply ingrained ideas about the fundamental nature of women may explain why Marks was given a commuted sentence, while McDermott was sent to the gallows. The jury recommended leniency for Marks because of her youth, but at 20 years old, McDermott was only a few years her senior. Susan E. Houston, professor emerita of history at York University in Toronto, suggests that in 19th-century Canada, the notion of a young woman being domineered by a more forceful man was a “much, much easier” story to swallow than the alternative.

“If you had to choose, then instinctively you would think because [McDermott] was the man that he was more in control,” Houston tells Smithsonian.com. “They played down the possibility that she could possibly have initiated this, or figured it out, or had any control over this young man ... And so therefore, he's the one who is the more culpable.”

“Nobody had any sympathy for McDermott,” she says.

Marks spent a total of 29 years in prison. It is not entirely clear why she was sent to the Provincial Lunatic Asylum in 1852. “There were various investigations of abuses in the prison, and punishments, and just how terrible the conditions of imprisonment were,” says Kendall. “So there's a sense that the conditions themselves were a contributing factor [to Marks’ mental health].” The Asylum superintendent, however, believed that Marks was faking her insanity.

After 15 months, Marks was sent back to the Kingston Penitentiary. During her incarceration, she impressed “many respectable persons” who petitioned for her release, Atwood writes in the afterword to Alias Grace. In 1872, Marks was finally granted a pardon. Records indicate that she subsequently went to New York. After that, all traces of her vanish.

To this day, Marks remains as enigmatic as she seemed in the mid-1800s. Was she a mastermind or a pawn? Cunning or simple-minded? An impressionable girl or a steely killer? The truth may lie at either end of these extremes or somewhere in between—in all likelihood, we will never know.

Before she disappeared from the historical record, Marks confirmed her version of events for a final time. Upon her release from the penitentiary, she was asked 27 “liberation questions” that were posed to all outgoing prisoners. “What has been the general cause of your misfortunes,” asked the 23rd question, “and what has been the immediate cause of the crime for which you have been sent to the Penitentiary?”

Marks was succinct in her reply: “Having been employed in the same house with a villain.”