Peter Jackson’s 1994 film Heavenly Creatures takes its audience on a voyage through the twisted fantasy world woven by of a pair of adolescent girl murderers. The movie ends with a scene that stabs an icy stake through the heart of innocence: the gruesome slaying of one of the girls’ mothers. The movie’s dreamlike atmosphere feels like a dark, warped fairytale, but it was based on a crime committed by two real girls in 1950s New Zealand: Pauline Parker and Juliet Hulme. The motive? Juliet was leaving for America, and Pauline’s mother was against Pauline going with her.

The subsequent trial argued for an insanity plea for the girls back in a time when a lesbian relationship could be seen as evidence of madness. The main concept behind their case was a psychological term called “folie à deus.” The phrase translates from French to English as “madness of two,” and refers to a psychosis shared by two people: their delusions permeating the barriers of self and nurtured by their intimated connectedness. It’s actually not that strange of a concept. Our worldviews are very often shaped by those close to us, even when they conflict with reality, but their can be a kind of shared consciousness between lovers, friends, or family.

In the 1950s New Zealand trial lawyers tried to argue for an insanity defense, claiming that the girls’ association with each other had made them go mad together, a situation defined by psychology as “folie à deus,” or shared psychosis. Before the grisly murder, the girls invented their own religion, making actors and celebrities they found to be sexy saints. They’d explore their sexuality with each other. These activities aren’t completely out of the ordinary or alarming, but the scale to which the girls took them seriously and sealed themselves off from the rest of the world definitely ended in trouble.

The girls, who wrote expansive stories together, worked together on a plot far more sinister than the fancies in their heads. The goal was to make the murder look like an accident. They decided to have tea with Pauline’s mother, Honora Rieper, and then take her on a secluded walk where they bludgeon her in the head with a brick tied in a stocking. They’d plan to make it look like she had simply slipped and fallen on the stone pathway, hitting her head, but it didn’t quite work out that way. Death didn’t come swiftly for Honora Rieper, and both girls took their turns with the weapon. It didn’t look like a simple slip-and-fall accident by the time Honora had passed away.

After spending some time in separate prisons, the two girls both went their separate ways and rebuilt their lives under new names. Juliet moved to England to be with her mother. She received the name “Anne” from New Zealand, and took her stepfather’s last name, Perry, as her own. In early adulthood she settled into Los Angeles, and there launched a successful paperback thriller writing career. Her books both titilate and mull over the good and evil of crime and murder. For decades her readers, and even her agent, had no idea that she had first hand experience with the act of killing.

It was only when Peter Jackson and his wife started working on Heavenly Creatures in the 90s that reporters started sniffing out the truth. Her literary agent was shocked, and so was the world. The idea of a crime writer being a secret murderer and having a kind of double life is as fascinating and titillating as it is a bit grotesque.

“I can see what she has to live with. When people can’t allow you to be something better than a murderer ever, then it’s a permanent sentence. There’s no getting out of that one. To survive, you’ve either got to shut that story out, or you’ve got to shut those people out,” Perry’s biographer Joanne Drayton said about Perry’s desire to “move on,” from inquiries about the crime. “It’s an accountability which, when the woman is in her 70s now, is just ridiculous. I think for her it seems like a relentless banging on about something that she knows was terrible, but she has put behind [her] and moved on, and done so much since.”

Although Perry shies away from interviews about the subject, she spoke a bit about her role in the murder in a documentary from a few years ago Anne Perry Interiors, which is a quiet, slow study of her secluded life in Scotland. In 2003, Perry gave The Guardian a bit more explanation about how she feels now about the events in 1954. “I felt I had a debt to repay,” she says about her role in the murder. “Pauline was the only one who had written to me when I was in hospital, and she threatened to kill herself if I didn’t help. She was vomiting after every meal and losing weight all the time. I am sure now she was bulimic. I really believed she would take her life and I couldn’t face it.”

She described her time in prison as torture, but felt it was what she deserved at the time. “It was cold, there were rats, canvas sheets and calico underwear. I had to wash out my sanitary towels by hand and they put me on to physical labour until I passed out.”

She calls prison “the best thing that could have happened,” because she was able to pay her karmic debt and repent for her sins. “It was there that I went down on my knees and repented,” she says. “That is how I survived my time while others cracked up. I seemed to be the only one saying, I am guilty and I am where I should be.” This act of repentance, of the tears she says she shed during this time, cleansed her, at least in her own eyes.

Although Anne Perry denies being in contact with her former friend, she lives just a few miles from her in Scotland. Pauline Parker now goes by the name Hilary Nathan. For a time she managed horse stables and a riding school in England, but relocated to Scotland after a local scandal erupted when people found out about her previous identity. It seems like an odd coincidence that the two women should be living so near each other after spending so many decades in different parts of the world. If they’ve wanted to reconnect, it certainly would be easy for them to do so. However, given the situation, it’s equally plausible that the two women have made a point to avoid each other’s presence despite their close proximities.