For an avatar of our current cultural appetite for accountability, truth-telling and radical moral reckoning, we couldn't possibly do better than Mildred Hayes, a grieving mother seeking justice and closure in "Three Billboards Outside Ebbing, Missouri."

Portrayed by Frances McDormand in a performance as ferocious and uncompromising as any of her career, Mildred turns out to be an alternately off-putting and deeply sympathetic guide through the world that writer-director Martin McDonagh creates. His movie fuses naturalism and hysterically pitched theatricality with sometimes uneasy, but bracing results.

McDonagh, known for such operatically profane, extravagantly brutal exercises as "In Bruges" and "Seven Psychopaths," doesn't stint on his signature flourishes: "Three Billboards Outside Ebbing, Missouri" is as dark as they come, a pitch-black, often laceratingly funny look at human nature at its most nasty, brutish and dimwitted. But he anneals the cleansing fire with moments of startling tenderness, using compassion to shock viewers the way other directors wield the dark arts of sex and violence.

As the movie opens, Mildred has not yet recovered from the sadistic rape and murder of her teenage daughter Angela, a crime that occurred seven months ago in the small Ozark mountain town of Ebbing. Spying three decrepit billboards on her way home one day, she hits on an idea to impel the local police chief, William Willoughby (Woody Harrelson), into action: She buys ad space on all three, fashioning a naming-and-shaming campaign asking him why the case is still unsolved.

Mildred's idea of avenging Angela inevitably has a cascading effect, not only with Willoughby - played with upstanding directness and pathos by Harrelson - but also by his dumb-as-a-rock deputy, Dixon, portrayed in an amusingly scurrilous turn by Sam Rockwell. Casting vanity to the wind, Rockwell affects an ungainly posture and unflattering haircut to play a racist, homophobic, supremely idiotic mama's-boy drunk on his own blunt-force power: If Mildred embodies fairness at its most extreme, Dixon is its opposite, a living, breathing symbol of unacknowledged, unearned privilege.

"Three Billboards Outside Ebbing, Missouri" is shot through with stinging, sometimes breathtakingly direct commentaries about racism and policing in a community that even though it's fictional, lies firmly within the orbit of Ferguson.

But McDonagh couldn't have anticipated the moment when his movie would arrive, a time when sexism in its most virulent forms has been revealed in a daily drumbeat of stories recounting unspeakable exploitation and abuse.

In that context, "Three Billboards Outside Ebbing, Missouri" is cathartic and occasionally disappointing. Thanks to McDormand's alert, responsive performance, Mildred's vigilantism possesses the purifying rage of a million Dirty Harrys rolled into one ruthless, indomitable package. Which makes it all the more dubious when McDonagh trots out stereotypically young, pretty, somewhat ditsy girls for comic relief. Then there's the inciting incident itself, a crime so heinous and hateful that Mildred's comically outsize response feels tonally off and, frankly, not credible, especially when it comes to her attenuated relationship with her teenage son, a largely ignored character played by Lucas Hedges.

If viewers can reconcile themselves with McDonagh's universe - a far more schematic, lurid, literary-minded and perversely taboo-challenging one than our own - "Three Billboards Outside Ebbing, Missouri" is brimming with subversive humor and deep satisfaction. It is usually to be found in Mildred's vicious, and vicariously delicious, encounters with all and sundry, from a mild-mannered Catholic priest and her abusive ex-husband (John Hawkes) to random strangers on the side of the road. (The gift shop where Mildred works is called Southern Charms, and she's anything but, cutting a grimly confrontational figure in a blue jumpsuit, no-nonsense bandanna and aggressive undercut beneath a don't-care ponytail.)

It's no surprise when McDonagh's funny, flamboyantly fallen world can be redeemed only by equally exaggerated acts of self-sacrifice. "Three Billboards Outside Ebbing, Missouri" is just the bitter pill the times call for, offered with a loving cup to make it go down just a bit easier.

If you go

THREE BILLBOARDS OUTSIDE EBBING, MISSOURI

3 stars

R, 115 minutes

Playing now at Madison Art Cinemas, Mystic Luxury Cinemas.