You know you’re watching an independent-minded movie when, midway through, it shifts from being a portrait of all-consuming grief and revenge to an examination of understanding and empathy against all odds.

Irish-born writer/director Martin McDonagh (In Bruges), with a mighty assist from Frances McDormand, makes motherhood righteous and terrifying, especially when that mother appears fully ready to scorch the earth after her teenage daughter has been raped and burned to death. McDormand’s rage and fury light the fuse that burns through the brutal yet darkly funny rustbelt tragicomedy Three Billboards Outside Ebbing, Missouri, which showcases not one but two Oscar nomination-worthy performances.

McDormand, in her best role since Fargo in 1996, plays Mildred Hayes, a divorced woman who scrapes together money to buy ad space on the three tumbledown billboards so that she can shame the local police chief, Bob Willoughby (Woody Harrelson, who is just right), for his bumbling force’s inability to find the monster who killed her daughter. The trail has gone cold. The billboards read in 10-foot-high black letters against a red background: “Raped while dying”; “Still no arrests? "How come, Chief Willoughby?”

It’s provocative stuff that rocks the little town where everybody knows everybody else’s business. But after seven months, the locals are ready to move on. They don’t want the case reopened, which was Mildred’s entire point. They want the billboards taken down, their own guilt expunged. Besides, they like the get-along, go-along Bob, a family guy with serious health problems; less so Mildred, who’d as soon spit in your eye as play nice. Mildred’s contentious, complicated relationships with pretty much everyone in the film make for some terrific scenes, including a few gems with her screwloose ex-husband (John Hawkes), their school-age son (Lucas Hedges) who wants his mother to stop her billboard campaign, and a pool hustler and potential suitor whom Mildred calls “the midget” (Peter Dinklage).

The film’s tone keeps shifting—tragic, comedic, acidly funny, knockabout farce—and none too smoothly, either.

What’s fascinating is how startlingly abrasive and caustic McDonagh writes Mildred, and how ferociously McDormand tears into her. Whether she’s lashing out at the Catholic priest who wants her to back down or navigating an excruciatingly awkward date with Dinklage—slowly revealing the layers of bile and vitriol that were there long before she lost her daughter—she’s spectacular. She has a single-minded, Greek tragedy-worthy wrath that approaches, say, Daniel Day Lewis’s performance in There Will Be Blood and Lee Marvin’s in Point Blank.

As crackling as Harrelson and McDormand’s quippy showdowns over justice and fairness are, they pale beside her confrontations with Officer Dixon (Sam Rockwell), a moronic, spineless deputy who listens to ABBA’s “Chiquitita” on his headphones, lives with his grasping, maniacal mother (Sandy Martin) and gets off on torturing black folks and prisoners. Out of loyalty to his police captain, he sets out to bring down Mildred. As Dixon, Rockwell isn’t just good here; he’s brilliant, shaded and masterful, especially every time he manages to make his character despicable and pitiful at the same time, which is often.

The film’s tone keeps shifting—tragic, comedic, acidly funny, knockabout farce—and none too smoothly, either. And the twisted roads down which McDonagh leads McDormand and Rockwell’s characters may outrage and surprise viewers, especially when things take a turn for the ridiculous (like Mildred’s firebombing of a building with people in it, a scene that could make even the most sympathetic viewer check out). Every once in awhile, Mildred’s character—or is it McDormand herself?—becomes a little too pleased with her own meanness. Still, for those who savor the slow burn of characters revealing themselves as deeper, stranger and perhaps more intertwined than we dreamed they were, McDonagh’s movie is a four-course meal, with writing so sharp and performances so bold and sly that it’s tough to leave the theater unmoved.

Three Billboards Outside Ebbing, Missouri

Read more of Stephen Rebello’s movie reviews here.