“You’re crazy,” sputters one of Carey Mulligan’s character’s victims when confronted by her sadistic revenge in Promising Young Woman. “You know, I really don’t think I am,” she says.

That you never lose sight of the righteousness to Cassie’s rage is the strength of Promising Young Woman, the hotly-debated directorial debut of Emerald Fennell.

Mulligan plays a withdrawn med-school dropout on a mission to avenge her best friend’s historic assault. By day she is a truly terrible waitress; by night she pretends to be black-out drunk at bars to lure self-professed “nice guys”, and teach them a lesson about consent they won’t ever forget.

With all the assiduous commitment to its concept of a music video, the film risks falling into the same trap of weighting style over substance as Fennell did with Killing Eve (which she took over from Phoebe Waller-Bridge for season two). But set aside the sugary-sweet, almost surrealist styling and Cassie’s bloody bravado, the truth of the film is quietly blistering and bitterly self-evident: there is little easier to move on from than a dead woman.

With the film’s release to UK cinemas delayed by the pandemic, much of the coverage has centred on a Variety review that suggested Mulligan had been miscast as the “apparent femme fatale”. Mulligan singled it out as sexist in other press: “I felt like it was basically saying that I wasn’t hot enough to pull off this kind of ruse.”

The critic, Dennis Harvey, has denied that that had been his meaning when he wrote that Margot Robbie would have been more plausible in the role of Cassie than the “chameleon” Mulligan.

At the very least, Harvey’s point was poorly articulated – but I would also argue that it was way off the mark.

Mulligan is not a shape-shifter in the way that we would think of, say, Charlize Theron. The quality she brings to this performance, and all others, is vulnerability – and that is central to what Promising Young Woman is trying to say.

The appearance of Seth Cohen, Superbad’s McLovin and Schmidt from New Girl in the “nice guy” roles shows how the film cleverly uses casting choices to play with audience expectations. By invoking the Suicide Squad’s Harley Quinn and the supremely self-assured Naomi on The Wolf of Wall Street, Robbie would have cut too much of an ass-kicking figure as Cassie.

Mulligan may be more steely than the average English rose, but she is far from unassailable; her being cast as the arbiter of revenge underscores the futility of the attempt. Promising Young Woman is not Kill Bill. If Mulligan is implausible as a woman seeking to single-handedly bring predators to justice, that may be because we know better than to expect them to face any consequences at all.

(Indeed, without wanting to give any spoilers, Cassie’s final triumph rang less true than the bleak but more obvious ending immediately preceding it. I wondered if perhaps the studio could not stomach concluding the film on such a grim note – for all its grounding in reality.)

The strength of Mulligan’s performance is that we never question her rage, even as we see the dismal limits to what it can achieve. Cassie’s anger is totally justified. But it doesn’t do anyone any good.