Cassandra "Cassie" Thomas (CAREY MULLIGAN) is a 30-year-old woman who lives with her parents, Stanley (CLANCY BROWN) and Susan (JENNIFER COOLIDGE). Once a promising med school student, she dropped out after something happened to her classmate and childhood friend, Nina Fisher, and now works in a coffee shop for Gail (LAVERNE COX).

Unbeknownst to any of them, Cassie spends many evenings picking up men in bars while pretending to be drunk and then putting them in their place once they're exposed for trying to take advantage of drunk women. Not surprisingly, her view of men isn't great, and she's initially suspicious of a former classmate, pediatric surgeon Ryan Cooper (BO BURNHAM), when he happens to stop by the coffee shop. But while she lets her guard down enough to start falling for him -- and vice versa -- his presence and then comments about some other former classmates of theirs spurs on a need for her to avenge what happened to her friend.

And thus, she sets out to meet -- under false pretenses -- former classmate Madison McPhee (ALISON BRIE) and Dean Elizabeth Walker (CONNIE BRITTON) -- neither of whom believed Nina's allegations about being raped while drunk by Alexander "Al" Monroe (CHRIS LOWELL) who's now a doctor and about to get married soon with Joe (MAX GREENFIELD) as his best man. And then there's Jordan (ALFRED MOLINA), the lawyer who got the charges against Al dropped.

As Cassie puts her plan into motion, her actions could potentially jeopardize her budding relationship with Ryan who's still unaware of what she's been doing and is now plotting.

It's interesting that no matter how evolved humans might think they are, some people still operate from basic instinctual impulses. While non-human predators will nearly always prey on the sick or weak, thankfully most folks won't do the same. But it still happens, often tied to sexual (mating) desire and the need to feel or exhibit dominance (being the alpha).

At the same time, and as happens in other species, those that survive such predatory or otherwise violent behavior often wish for revenge. Granted, that's morphed into additional levels on the human side, but its basic underlying principle is to make it known that such victimization will not be allowed again or will face severe consequences.

On that human side, though, things get more interesting when others who either witnessed such an attack or later heard about it (from the victim or third parties) feel the need to avenge those who were wronged.

That's the fascinating backdrop of "Promising Young Woman" where Cary Mulligan plays Cassie, a now 30-year-old woman whose med school life was upended in the past due to something that happened to her classmate and lifelong friend. She dropped out, moved back in with her parents (Jennifer Coolidge and Clancy Brown), and works at a coffee shop (for her boss/best friend played by Laverne Cox).

But her real vocation is spending evenings pretending to be intoxicated in bars, allowing lecherous men to believe they can take sexual advantage of her, and then putting them in their place while suddenly showing she's stone-cold sober and calling out their behavior.

Not surprisingly, she has a dim view of most men and thus hasn't been romantically involved with anyone for years. But then a former classmate -- Ryan (Bo Burnham) -- comes into the coffee shop, recognizes her, puts his foot in his mouth, and immediately shows how sorry he is -- and her icy demeanor begins to melt a bit. As does her heart.

It turns out, however, that he knows some of the people from that past event, and upon speaking their names, that sets off a new level of "avenging angel-ism" in Cassie. Natch, she doesn't inform him of her moonlighting behavior but then sets out to step up her game in terms of going after those directly responsible or tied to what ultimately happened to her friend.

It's not a particularly novel idea (beyond switching the gender of the vigilante, although that has been done before -- notably 2005's "Hard Candy") and it makes one wonder why she never went after the guilty before now rather than racking up an impressive list of strangers' names that she keeps in her "I humiliated" notebook compilation.

That aside, Mulligan's captivating performance is what makes the film -- along with writer/director Emerald Fennell's script and sure-handed direction -- work so well. Her character's guilt over what happened and never-ending desire to make things "right" and prevent the same from happening to others has overwhelmed her and the actress makes that believable and then some.

I won't give away how things ultimately play out, but it's fitting, I suppose, given her obsessive, self-destructive behavior that ends up temporarily softened a bit by a guy who might just prove her view of men as wrong, or at least flawed. Likely to appeal to women with revenge fantasies based on something similar happening to them or someone they know or just the way some people still behave like animals, "Promising Young Woman" rates as a 6.5 out of 10.