The germ of the idea for “Promising Young Woman” first lodged itself in Emerald Fennell’s mind six or seven years ago, at a dinner party she and her roommates were throwing for some old college friends in London. Everyone was sitting around the kitchen table, eating pasta, when one woman happened to mention a creepy encounter she’d had with a guy on the tube on her way over. The men at the table were shocked. The women were shocked that the men were shocked. What world did they live in? Apparently not one filled with creeps who followed you home, or groped you on public transport, or catcalled you and turned nasty when you ignored them.

In other words, the usual. But the men at this party might as well have been walking through a wardrobe into a land of perpetual winter. As women regaled the table with one gruesome story after another, gleefully besting one another’s floridly crappy experiences, they were shocked by the relentlessness of it all, and by the gallows humor and resignation in the women’s response. “They were just staggered,” Fennell told me when I met with her last winter. “And these were just the milder things.” One man said he grew up thinking everything was fine, and was just now realizing it was only fine for him.

The experience was an eye-opener for Fennell as well. “Their surprise was so interesting,” she said. She suspected men would not be so unaware of women’s experiences if women weren’t culturally shamed into “laughing off” or “being cool with” their trauma — helping to create a fairy tale in which everything really was mostly fine, and bad things only happened occasionally, to girls who probably did something to deserve it. What made this striking was not the actual events the women were describing, which were too quotidian to be horrifying; it was seeing how readily the culture enabled and normalized this stuff, making women feel uncomfortable or embarrassed for talking about it honestly.

The film that emerged from this realization, “Promising Young Woman,” is Fennell’s debut as a feature director — a ruthless, pitch-black story of revenge set in an off-kilter, fairy-tale world. Carey Mulligan plays Cassie, a young woman who dropped out of medical school after a traumatic incident the film does not initially reveal. At 30, Cassie still lives with her parents and works as a barista in a coffee shop. But her real mission in life, which she pursues with singular dedication, is to confront people who think of themselves as blameless with the truth about their behavior. Every week she dresses up for a night out — sometimes in business attire, other times in more revealing outfits. She goes to bars and pretends to be blackout drunk. Invariably, a man comes to her rescue. Invariably, he takes her home and tries to have sex with her. Then things take an unexpected turn.

Fennell started writing after thinking over all the conversations she’d participated in about alcohol and consent — all the rollicking stories guys told about hitting on drunken girls, or getting them drunk to “loosen them up.” None of this was taboo when she was younger: “It was all completely normalized by all the American ‘raunch era’ films and TV that everyone watched,” she told me. “Drinking was part of seduction culture — and if people couldn’t remember things, it was often met with an eye roll.” Fennell questioned that logic. If having sex with a girl who was blackout drunk was nothing to feel bad about, then a man wouldn’t feel guilty if she turned out not to be drunk, would he? It made her wonder. “What if I went to a nightclub and pretended to be really, really drunk, and somebody took me home, and then just as they were removing my pants, I revealed I wasn’t drunk?” An image formed in her mind of a woman sitting up in bed, suddenly sober, and asking, “What are you doing?” She later described this very scenario to a producer. “I said, ‘And then she sits up, and she’s not drunk!’ And he went, ‘Holy [expletive], she’s a psycho!’”

This was the reaction she’d hoped for. “The reason it feels so uncomfortable is because the person who’s doing it knows it’s wrong,” she said. “That’s why they freak out. Everybody thinks of themselves as a good person — so what happens when someone comes along and shows you that you’re not?”

With her long, wavy blond hair and flouncy dresses, Cassie looks like a romantic-comedy heroine, or like the good girl in a film noir, but she radiates white-hot rage, and not even the stifling artificiality of her parents’ house, with its pink wall-to-wall carpeting and passive-aggressive suburban rococo furniture, can smother it. From the film’s opening image — a hilarious, slow-motion sequence of paunchy, khaki-clad office dudes on a dance floor, gyrating and slapping their own butts — “Promising Young Woman” subtly skewers gender conventions and double standards, and as the movie progresses we start to piece together what is happening: Cassie is trying to redress an injustice that was swept under the rug, by not allowing anyone to forget.

Fennell has been scrupulous about crafting the mechanics of Cassie’s revenge: “She doesn’t entrap anyone. She never says yes, she never says no. She just exists. She says, ‘I’ve lost my phone,’ and then they do all the talking.” What you see, Fennell said, “is a man thinking he’s got a rapport with a woman, which I think happens a lot. It’s just that he hasn’t noticed that she’s not said a word.” The moment Cassie reveals that she is conscious of what is happening is, for that person, the ultimate threat: She forces them to confront themselves. “Isn’t that the worst thing?” Fennell laughed. While pitching the movie, she would joke that most people would rather be shot in the knee than be shown who they really are. “That’s our worst nightmare,” she said. “It’s what makes Cassie frightening — much more frightening than a knife-wielding maniac. Much more devastating, really.”

I met Fennell for tea last February, in the library of the Soho Hotel in London — a cozy, faux-bookish setting where, moments before she joined me, a man at a nearby table loudly and graphically debriefed two others on some torture instruments he’d recently had the chance to inspect. Fennell arrived two minutes late, in jeans and an oversize, fuzzy, bright pink sweater, apologizing profusely. She looked as if she could have stepped directly off the set of her movie, in which she has a cameo as a video blogger giving a “Blow Job Lips Makeup Tutorial.” Fennell herself is compulsively, hilariously self-effacing — a trait she attributes partly to being female and partly to being English — but her good friend Phoebe Waller-Bridge, of “Fleabag” fame, whom she first met on the set of the film “Albert Nobbs,” calls her “the most stylish person I’ve ever met. Not just in her work and her appearance, but in her spirit, how she speaks, how she carries herself.”

Fennell is highly attuned to presentation. When I commented on the brilliance of Nancy Steiner’s costume design for her film, which makes everyone look like a character in a Hallmark movie of the damned, she spoke about the ways women know how to use clothes, hair, makeup and voice to hide their anger and trauma. “There are lots of people who hide it by putting on really accessible, really sweet, really unthreatening — oh … ” She stopped. “I just realized I’m wearing an enormous jumper.”

Tonally, there is a similar tension at play in Fennell’s movie. Her work tends to feel, in general, like an enormous, fuzzy pink jumper wrapped around a dagger. As one of the film’s producers told me, “Emerald would describe this as ‘poison popcorn,’ which I think is a great term for it.”

Fennell may be better known as an actor and writer than as a director — especially given her role on “The Crown,” a huge hit whose latest season included the marriage of Prince Charles and Diana Spencer. As Camilla Parker Bowles, Fennell plays a character with an upbringing she’s familiar with — “I’m basically playing a chain-smoking posho standing in a corner making cutting remarks,” she said. “So it’s not a stretch” — who finds herself cast as the villain in a fairy tale, which, in reality, was anything but. “I was drawn to Camilla because she struck me as a normal person sucked into a completely extraordinary circumstance,” Fennell said. This comes across in her performance, which hovers between amusement and disbelief.

The time period covered in this season of “The Crown” roughly corresponds to the years just before Fennell was born, in 1985. She grew up in Chelsea, in a flat that was eventually joined to another to form a house. Her father is the celebrity jeweler Theo Fennell, known for his intricate, often dark and funny one-of-a-kind pieces, like “opening rings” that hinge back to reveal magical, fairy-tale worlds (a Mole and Toad piece inspired by “The Wind in the Willows,” a Colosseum with a dead gladiator in it). Her mother, Louise, worked in fashion and as a photographers’ agent before writing, in her mid-50s, her first book, a satire of celebrity called “Dead Rich.” Emerald’s sister, Coco, is a fashion designer. Elton John and Andrew Lloyd Webber, at whose offices we would meet for a second time, are friends of the family.

Fennell was educated at Marlborough College (the boarding school that the Duchess of Cambridge, Kate Middleton, also attended) and studied English at Oxford, where she performed in plays and was spotted by an agent. She auditioned for what she thought would be a one-episode role in the BBC drama “Call the Midwife,” but her character, Nurse Patsy — a redheaded lesbian with a blunt demeanor and a traumatic past — remained on the show for three seasons. In between those seasons, Fennell wrote books, one for each hiatus: two children’s stories set at a creepy boarding school, and one adult novel, “Monsters,” a black comedy about two kids who are delighted to find a dead body on the beach.

She works, says Waller-Bridge, “like a bloody Trojan. She’s been working on about 10 projects at once since the day I met her.” She has been known to work on writing projects even after 14-hour days on television sets as an actress. She shot “Promising Young Woman” over 23 days in Los Angeles, while seven months pregnant. After Waller-Bridge’s departure as the showrunner of “Killing Eve,” Fennell joined the writing staff for Season 2 and, after a few months, was promoted to head writer and co-showrunner, eventually winning Emmy and Golden Globe nominations for her work.

This prodigious output would be remarkable even if she weren’t just 35 (or 34 when we first met). At the time, she was on a short break from shooting “The Crown.” She was also promoting her movie and writing the book for Andrew Lloyd Webber’s upcoming musical “Cinderella,” expected to have its premiere in 2021. When Lloyd-Webber first approached her about collaborating, she thought: “ ‘Cinderella’ — there’s not really much one can do.” Then she thought: What if Cinderella were a normal person who was forced to live in a fairy-tale world? We’re used to the story of the girl who gets made over and rescued, but what if, instead of the transformation being the best thing that ever happened to her, it was the worst? She pictured a woman who didn’t mind being who she was — “and then, suddenly, they’ve been made to mind.” Her “Cinderella” is the story of a real girl in a fairy-tale world that expects her to annihilate herself to meet its demands.

Fennell grew up reading stories of beautiful cheerleaders, of gorgeous, glowing, unconscious girls. But her real loves were Nancy Drew books, Shirley Jackson, Patricia Highsmith, Daphne du Maurier and the Brontës. (“The Brontës! The greatest!” she wrote to me later. “All of them — except Branwell, obviously.”) “All the stuff that I love — all the Victorian female novelists, the perverted domestic, the madwoman in the attic — all that stuff, in a way, is what I would love to be able to do,” she says. Recently she’s been reading Hilary Mantel, whose work she finds can be “very visceral and very feminine, horrifying in a way I’ve never ever experienced.” Literature, she says, is full of fascinating, frightening women, “but when it comes to television and film — I suppose because our preoccupation with the women in that media is still based on the way they look — we don’t see those characters so much. These kind of weird old ladies or pervs or voyeurs. We don’t see female losers at all.”

One day in the early 2000s, when Fennell was a teenager, she was at a cash machine, wearing a crop-top that exposed her pierced navel, and noticed an elegant, well-dressed woman hovering uncomfortably nearby. Finally the woman spoke to her: “I’m so sorry,” she said. “I didn’t know whether to tell you or not, but you’re going to die of stomach cancer before you’re 30.” “I said, ‘What?’” Fennell remembers. “And she said, ‘I just thought you should know.’ Then she walked away.” Fennell was stunned, but the casual savagery of the gesture — the subtle, underhanded violence of it — impressed her. To this day, she thinks of it every time she has a stomachache. “Isn’t it so clever to pretend to be a kindly citizen?” she laughs. “I just thought, That’s it. That’s what it’s like. That’s what it’s like to be an angry, frightened, mean woman.” Years later, she included it in a short film, “Careful How You Go,” which consists of three vignettes depicting three moments of psychological violence and recreational sadism. “I guess she’s my muse,” Fennell said. “That cruel, cruel woman.”

In the past five years or so, after decades of seeing women subsumed into highly regulated, rigidly prescribed roles, we’ve seen an explosion of dark, uncontained, shockingly human female characters. There’s a sense, Fennell told me, that the types of stories she wants to tell are “new” or of-the-moment in film and television, but she believes they have always existed. They’ve just been walled in, closed off, “like those anchorites” — medieval ascetics — “who used to build themselves into the walls of churches and see insane, terrifying visions and write about them.” What is fresh is that they are appearing in films and on television. Waller-Bridge’s “Fleabag,” Michaela Coel’s “I May Destroy You,” Aisling Bea’s “This Way Up,” Pamela Adlon’s “Better Things,” Abbi Jacobson and Ilana Glazer’s “Broad City” and, more recently, Maya Erskine and Anna Konkle’s “PEN15” and Lucy Prebble and Billie Piper’s “I Hate Suzie” — this is an emergent mini-canon of tales from the other side, from behind the veil of decorum. “We’re only just getting to the stage, some of us, to tell them,” Fennell said. “I feel like there’s a backlog of stuff.” They aren’t new stories so much as alternate ones — subversions of the official story, secret histories, gnostic texts. “They’re the underworld,” she said.

Fennell has been encouraged, recently, to see shades of this underworld — works marked by senselessness, chaos, the ease with which savagery can be cloaked in banality, all the repressed darkness and gallows humor that women use to cope — all around her: in Alice Lowe’s slasher film “Prevenge,” in Julia Davis’s filthy, Victorian-themed black comedy “Hunderby” or her hugely successful, also hugely filthy, podcast with Vicki Pepperdine, “Joan and Jericha,” in which they dispense advice as “two women for whom nothing is too disgusting. In fact, everything should be more disgusting. But also women are always wrong — so every woman who emails in, whatever the email, no matter how terrible or vile her partner, it’s always the woman’s fault.”

Fennell told me a story about visiting the White Cube gallery in London, where she became enraptured by “a very weird sculpture of a woman having sex with a huge tentacled creature, or being murdered by it, or something.” She remarked to a gallery assistant how much she liked it. He told her there had been mixed reactions to it — “But do you know who loves it? Women.” Considering how women have embraced the surge in dark, realistic portrayals of contemporary female life, this is not surprising.

There is something about the way the world relates to women that is bound to breed darkness — even if that darkness is sub rosa, hidden under blond curls and pretty dresses. This unvarnished darkness should not be confused with earlier, often studio-driven attempts at girl-themed “raunch culture.” It is coming from inside the house, reflecting a certain kind of smart, sensitive, reflexively caustic woman’s view of a culture that seems to insist on keeping her hidden from view, and subbing a compliant fembot in her place. As Fennell observes, it’s much more comfortable to imagine women are sweet and happy than face the fear they might want to hurt you. Cinema is full of stoic, gun-toting, “empowered” female avengers, but “that’s not how it works when women are angry and upset and traumatized,” she said. Cassie’s refusal to forget is more threatening: a constant, unendurable rebuke to those around her. “It was important that there was another path for her,” Fennell said. “And that we see how smooth and soft and well-lit that path is, versus the other one, which is so bleak.” Nothing threatens a culture of complicity more than self-sacrifice.

After watching “Promising Young Woman,” Fennell told me, she noticed that a male friend of hers looked upset. “I said, ‘Are you all right?’ And he said, ‘You’ve been watching everyone.’ And I was like, ‘Yeah.’ I don’t want to be cruel. I want to be honest.” She paused. “Let’s talk about it.”