The Tale is based on a short story Fox wrote when she was 13, narrated verbatim in the film: As a young girl, she attended a horse-riding camp where her instructor, Mrs. G (Elizabeth Debecki), lured a shy Jenny into a sexual relationship with her lover Bill (Jason Ritter), a fortysomething running coach. The movie opens with an adult Fox besieged by frantic calls from her mother (Ellen Burstyn), who’s just stumbled across the story. Shockingly, the director dismisses her mom’s alarm as the prudish overreaction to a long-ago “relationship” with an older “boyfriend.”

It’s a truthful but destabilizing point of entry, not least for the film’s star. “To try to understand someone who, at 40, is defending this narrative was very hard for me,” says Dern, 51, who plays Fox with the tenacity of an oak branch. “At times, I felt outrage. She was defending the story of this lonely child who was deeply seen as special by someone. That is the part I had to emphasize – and it was the part that just made me sick.”

As portrayed by Debecki, Mrs. G is a European sophisticate, and Ritter’s Bill her handsome cohort. Together, they appeal to young Jenny, treating her like a grown-up and inviting her to share in the secret of their bohemian love affair – it’s easy to see why she’s initially drawn to them. “Funny how you live with people in your mind – inside of you they’re always the same,” the elder Fox recalls in one of several insightful voiceovers. “You live with them happily, never wanting anything to change.”

A veteran documentarian, Fox began thinking about this project while finishing her episodic, six-hour portrait of female sexuality, Flying: Confessions of a Free Woman. Yet despite her background in nonfiction filmmaking, she opted to turn The Tale into more of a meta-text, revisiting her past trauma as part of a grueling exercise that explores how, in the infamous words of Joan Didion, we tell ourselves stories in order to live. “I had to blow the lid off traditional storytelling,” Fox says. “It’s about unravelling denial, using myself as the red thread.”

She also reveals in flashback how she was escaping a home life where she was both overlooked and overparented, lost among her many siblings but forbidden the harmless fun of friends’ sleepovers. Meanwhile, Ritter’s Bill doesn’t immediately register as a boogeyman – and this preference for nuance over easy-to-recognize nightmares is what makes The Tale so genuinely groundbreaking. “Most of the time the films I’ve seen portray abusers as outlaws and outliers, and that’s simply not true,” she says. “They don’t portray the child’s complexity of feeling about the perpetrator. The reality is 98 percent of all sexual abuse happens by people whom the child knows and feels comfortable with.”

Instead, Fox conveys the devastating force of Bill’s predatory behavior through a profoundly more powerful technique. As the film begins to sort through Fox’s memories, Dern’s character remembers herself as an adolescent (played by Jessica Sarah Flaum) on the brink of womanhood. But when she consults a photo album, she finds a flat-chested, pre-pubescent girl (played by Isabelle Nélisse) with a face still ringed by baby fat. Suddenly, the scale and depravity of the abuse is brought into sharp relief. (A title card at the end notes that all sex scenes were filmed with an adult body double, though it doesn’t make them any easier to stomach.)

The swap in casting makes the story more universal: Who among us hasn’t stumbled across an old photo only to be confronted by the strangeness of our youth? “If my 13-year-old self met me now, she would hate me,” says Fox. “The more I tried to see myself and write about her, the more I realized that I didn’t really know who she was anymore. I had become a different person.”

Fox refers to these narrative tweaks as refrains, deployed throughout the script in service of untangling memories: an iPhone ping interrupts a flashback, the shadows cast by a fire flicker across characters’ faces in a room that is later revealed to be cold and dark. In imaginary interviews, Dern’s Fox angrily cross-examines her younger self. “You’ll never get married, you’ll never have children,” she says. Nélisse’s stubborn Jenny responds: “I’ve got something no one else does. I’m not the victim of this story. I’m the hero.”

Such resistance to tying tidy emotional bows on everything was what attracted Dern to Fox’s script, which was passed to her by their mutual friend, director Brian De Palma. “One of the ways we survive – and say, ‘Oh, my god, I’m so lucky, I don’t know how this person survived that’ – is we tell ourselves that what we went through wasn’t bad,” says Dern, who has become a vocal supporter of the Time’s Up movement. Last year, during an Ellen appearance, the Big Little Lies star confessed to having subconsciously denied her own assault, at 14. Like Fox, she resisted the narrative of victimhood. It was only after she shared the experience with her mother, Diane Ladd, that the latter labeled it correctly.

And Dern, who grew up surrounded by show business, is in a unique position to address her character’s well-worn defense in the movie. “It was the Seventies!” Fox tells her partner (played by Common), a feeble excuse at best. “Of course, we were more permissive back then – we were more permissive five months ago,” says Dern. “We never questioned filmmakers with their young wives or fashion photographers with their young muses. I’ve been in rooms where people are like, ‘Well, yes, she was 15 or 16 but she looked 22, and she was already working.’ It was what it was, and now we’ve trained our eyes to look at things differently.”

Dern is grateful for Fox’s bravery in not only adapting her experience but also using her own name. “This story gets told because the writer-director was that child,” she says. “It has a level of starkness and intimacy. If it wasn’t her story, I don’t know that she could be that bold because inevitably people would go, That’s too egregious, that’s too in-your-face.” For Fox, there was never a question of fictionalizing anything, other than what was necessary to protect certain identities. “I would be terrified that people would call for those scenes to be out,” she says. “Most films, when it comes to the actual act, a door is closed and we can look away from the horror. For me it was important to show just how awful it really is. It needed my authority.”

Still, Fox is well aware how potentially triggering the material is, which is why she’s grateful the movie was acquired by HBO after premiering at this year’s Sundance Film Festival. “I just wasn’t sure people were going to leave their home for it,” she says. “Many people prefer to see this quietly and then talk about it.”

To that end, The Tale‘s HBO debut will coincide with coordinated screenings with Planned Parenthood and Joyful Heart Foundation, and the movie’s official site will feature resources, including the RAINN hotline, to further dialogue and outreach around the issue of sexual abuse. But despite its adjacency to activist causes, the movie should not be filed away as a public-service announcement. It’s an artistic triumph in terms of the way stories – about women, trauma, and selfhood – are translated to the screen. “People can’t wait to see this film,” says Dern, who spoke on the morning after the recent Met Gala, where she had a memorable conversation with an actress who was in tears talking about the movie. “I don’t think that could have happened if we weren’t in this zeitgeist.”

Certainly, Fox is grateful for the timing in the sense that it might encourage more people to embrace The Tale. “This was my story,” she says. “I wanted to make sure people couldn’t deny it.”