Carey Mulligan, front, and Bo Burnham are shown in a scene from "Promising Young Woman." (Submitted photo / Focus Features)

Cassandra Thomas wanted to be a doctor.

Instead, she becomes a teacher, re-educating “nice guys” one uncomfortable encounter at a time.

Writer-Director Emerald Fennell’s “Promising Young Woman” is equal parts vicious and wickedly funny, using its humor like a blunt object to hammer home a story about society’s tacit acceptance of sexual assault under the guise of date rape and the lingering, long-term impact on its victims.

Cassandra spends her days working in a coffee shop and her nights hanging out in bars, playing the role of the woman who has had too much to drink. She baits the hook, waiting for the assistance of a “nice guy” who offers to get her a ride home, but it usually turns out to be his home, where he proceeds to take advantage of the nearly incapacitated woman … until a stone-cold sober Cassandra stops him and teaches him a lesson about taking advantage of drunken women.

Fennell is coy about what Cassandra is doing to the men and what set her on this path. It’s clear Cassandra dropped out of medical school, and something ugly happened to her best friends and classmate, Nina Turner, who is no longer around.

Some details aren’t hard to figure out in advance; other arrive with a shock equal to the one Cassandra gives the men.

While the guys are random targets, there are other more elaborate plots involving people directly connected to whatever happened in med school, but those shouldn’t be ruined here.

After a decade as an actor (“The Crown,” “Call the Midwife”) and writer (“Killing Eve”), Fennell is impressive in her feature directing debut. She contrasts the tone of the story with a candy-colored visual palette and displays a sure hand in dealing with the subject matter in a darkly comedic way.

So many black comedies fall apart about halfway through. Either the directors — or the studios that greenlit them — lose their nerve, and the story unravels. Fennell nails the landing.

It helps having Carey Mulligan in the role of Cassandra. It’s a truly dazzling performance, one that’s already attracted some critics’ awards and is sure to attract more.

Mulligan revels in those moments of power when she turns the tables on her male attackers, but she also conveys how temporary and hollow those moments are. She’s a wounded soul on a self-destructive path.

Some of her best scenes involve her relationship with Bo Burnham as Ryan, another med school classmate who now is a pediatric surgeon. Her wariness and discomfort achingly are evident as she slowly tries to have a relationship with someone who seems like a genuinely nice guy.

Fennell also has written juicy scenes for a host of familiar actors who make brief but memorable appearances — Connie Britton as a medical school dean, Alfred Molina as an attorney who specialized in defending men accused of rape, Molly Shannon as Nina’s mother, Alison Brie as a former classmate and Adam Brody and Christopher Mintz-Plasse as a couple of the “nice guys” Cassandra encounters and Clancy Brown and Jennifer Coolidge as Cassandra’s supportive but frustrated parents.

The movie also uses its soundtrack effectively. Anthony Willis’ cover of Britney Spears’ “Toxic” makes the song as menacing as the “Psycho” theme, and its use of Paris Hilton’s “Stars Are Blind” becomes one of the movie’s silliest and most joyous moments.

“Promising Young Woman,” which opens in theaters on Christmas day, is reminiscent of movies like “Heathers” and Alexander Payne’s “Citizen Ruth,” which realize comedy can cut deeper than drama when dealing with serious subject matter.

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WHAT: “Promising Young Woman”

STARS: Carey Mulligan, Bo Burnham, Clancy Brown, Jennifer Coolidge, Laverne Cox, Alison Brie, Chris Lowell, Max Greenfield, Molly Shannon, Alfred Molina and Connie Britton

STORYLINE: A former medical student seeks revenge for a former classmate and looks to re-educate “nice guys,” one uncomfortable encounter at a time.

DIRECTOR: Emerald Fennell

RATING: R for strong violence including sexual assault, language throughout, some sexual material and drug use.

GRADE: A