Is it a #MeToo-era revenge thriller? A 21st-century “Fatal Attraction”? A candy-colored feminist polemic, to strains of Paris Hilton and Britney Spears? A black comedy? A comic tragedy?

None of the above — and all of the above. Which is to say: a film as bracingly original as Emerald Fennell’s “Promising Young Woman" defies labels.

Here’s a couple good adjectives, though: Startling — the only way to describe Fennell’s vision and execution in this, her directorial debut. And, sensational — a word that only begins to capture Carey Mulligan’s lead performance.

We’ve seen Mulligan command the screen before, and witnessed her consistently fearless stage work. But the tonal balancing act she conquers in this audacious, disconcerting film is something new entirely, and one wonders how many actors could even dream of pulling it off. It’s been a decade since Mulligan, now 35, received her only Oscar nod, for “An Education.” Let’s hope Oscar voters will be well educated by this latest effort.

We first meet Cassie (Mulligan) sprawled on the red banquette of a club, near the dance floor, her business attire askew. She’s clearly blind drunk. A group of office guys stare, tsk-tsking. One of them — nice, wholesome Jerry (Adam Brody) — decides to rescue her.

No harm if the “rescue” includes stopping at his apartment for a quick drink, right? As she drunkenly murmurs, “What are you doing?” he starts to undress and caress her, assuring her she’s “safe.” Suddenly, she asks the question again — stone-cold sober. It’s all been an act. Oops!

This is how Cassie, a former medical student, spends one night a week, teaching a lesson about consent and sexual misconduct to so-called “nice guys.” When, the next morning, we see a troubling red streak down her leg, we don’t exactly know what she’s done. But we know it’s payback, for something that once happened to her adored school friend Nina.

It’s this undefined trauma that forced Cassie to drop out, derailing her career. She now lives at home with her worried parents (Jennifer Coolidge and Clancy Brown, both terrific) and works at a coffee shop. Where, one day, a sweet, self-effacing, slightly nerdy former med-school colleague shows up, eager to reconnect.

At first Cassie resists, but she’s clearly attracted to Ryan (a tremendously appealing Bo Burnham) and we root for them immediately. Cassie slowly lowers her guard, and the two share a charming scene in a pharmacy, doing impromptu karaoke to Hilton’s “Stars Are Blind.” (Pharmacy karaoke: it could become a thing.)

This could be Cassie’s route back to normalcy, and she resolves to move forward. But then she hears that the main perpetrator of the trauma involving Nina is back in town, planning to get married. This propels her into an ever more dangerous set of encounters, all in the name of avenging her friend.

The stellar supporting cast includes Alison Brie as a former school friend, Connie Britton as a med-school dean, Alfred Molina as a remorseful lawyer, Laverne Cox as Cassie’s sympathetic boss, Christopher Lowell as a troubled figure from the past, and others, all excellent.

As scenes with Brie’s and Britton’s characters attest, it’s not only men who have retribution awaiting. But they bear the brunt of Cassie’s anger, especially those who go through life convincing the world — and themselves — what “gentlemen” they are.

The tone shifts radically from one moment to the next, and humor is a regular companion to mayhem, pain, even violence. That brings us to the wild and harrowing ending. It’s an ending that may not be expected — well, it’s definitely not expected — but Fennell has said it was the truest way to end a real story of female revenge, not a comic-book version.

You may agree, or not. No matter: the final act is riveting. I’ll admit to having been unable to remain seated during the last 25 minutes, pacing the room and shouting “No!” in disbelief at my laptop (this might be more difficult if you’re in an actual theater.)

When I watched a second time, I was able to focus less on the shock of the ending and more on the crafting of those final minutes, from the stunning monologue Mulligan gives to explain her motives, to the escalating suspense, to the chilling choice of music.

Startling, indeed. And that Mulligan performance? Sensational. Those are the only labels we need.

“Promising Young Woman,” a Focus Features release, has been rated R by the Motion Picture Association of America “for strong violence including sexual assault, language throughout, some sexual material and drug use.” Running time: 113 minutes. Three and a half stars out of four.

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Definition of R: Restricted. Under 17 requires accompanying parent or adult guardian.

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Follow Jocelyn Noveck on Twitter at www.Twitter.com/JocelynNoveckAP