In the first act of the awkwardly titled but brilliantly executed “Three Billboards Outside Ebbing, Missouri,” we think we’ve got a handle on the main characters and what they’re all about, including:

• Mildred Hayes (Frances McDormand), a grieving divorced mother simmering with rage because the local police haven’t found a suspect or even unearthed a promising lead since her teenage daughter Angela (Kathryn Newton) was kidnapped, raped, set afire and murdered.

• Mildred’s teenage son Robby (Lucas Hedges), a smart (and smartass) kid mourning his sister — but also deeply embarrassed by his mother’s increasingly bold and even violent antics.

• Police Chief Willoughby (Woody Harrelson), who is gruff and foul-mouthed, but also clever and a good family man.

• Jason Dixon (Sam Rockwell), an alcoholic, thick-witted, near sociopathic cop who lives with his mean old racist witch of a mother.

• Red Welby (Caleb Landry Jones), the smirking, perpetually jumpy local advertising man, who gladly accepts Mildred’s cash offer to paint a message on three billboards that are, well, just outside the town of Ebbing.

We think we know these people, because the writer-director Martin McDonagh (architect of “In Bruges,” one of the best dark comedy capers you’ll ever see) has done a masterful job with the script and with his visuals (North Carolina stands in for Missouri), and because the cast turns in perhaps the best ensemble work of any movie this year.

And yet just when we believe we know these people and we have a pretty good notion of where the story is going, we are hit with surprises both small and enormous. The fates of multiple characters are not the fates we expected.

Following the path of “Three Billboards” is a little like driving down an unfamiliar road in beautiful but forbidding country late at night, and alternately marveling at the scenery and gripping the steering wheel tightly when yet another steep drop or sudden change of direction presents itself.

About those billboards. Painted in bold black letters against a blood-red background, the message reads:

RAPED WHILE DYING
AND STILL NO ARRESTS?
HOW COME, CHIEF WILLOUGHBY?

Everybody in and around the town of Ebbing knows the billboards are referencing the horrific murder of a teenage girl some eight months ago, and even though Red Welby says he can’t divulge the identity of the individual who paid for the billboards, everybody knows it had to be Mildred.

Willoughby is ticked off, Mildred’s ex-husband (John Hawkes) is mortified and Dixon the cop is ready to take matters into his own hands. Meanwhile, Mildred goes on the local news and tells her story. (The depiction of an ambitious, overly dramatic, semi-clueless, extremely local TV reporter is a brutal but spot-on takedown.)

The mystery surrounding Angela’s murder is always lurking on the edges of “Three Billboards,” but somehow McDonagh has taken the bleakest of subject matters and treated it seriously while also serving up one of the best dark comedies I’ve ever seen. In scene after scene, McDonagh and that outstanding cast deliver small chuckles and hearty laughs that spring authentically from the situations at hand.

The middle-aged, slightly paunchy, hard-edged Willoughby is married to the beautiful Anne (Abbie Cornish), who speaks with a trace of an Australian accent. They have two wonderful little girls. The Willoughby we see at home is a far cry from the Willoughby cracking heads as the police chief.

Then there’s Mildred’s ex-husband Charlie (John Hawkes), now shacking up with 19-year-old Penelope (Samara Weaving, hilarious in a small part), who says she’s reading a book about polo or is it polio. Which one is about the horses, asks Penelope.

And in one of the most verbally vicious and one of the funniest rip jobs ever seen on film, Mildred politely listens to the local pastor’s suggestion she take the down the billboards and frequent Mass more often, and then proceeds to explain in cringe-inducing language why the good father can take his suggestions and, um, leave her house.

Harrelson and Rockwell are the standouts in the supporting cast, but this movie is McDormand’s to own, and of course we shouldn’t be surprised that she carries the story with fierce, foul-mouthed, abrasive, ferociously loving heart. Mildred would be the first to admit she’s not an easy person. Often we’re not convinced she’s even a good person. (Peter Dinklage’s James, a local car dealer with a crush on Mildred, finally tires of her needlessly rough treatment of him and lets HER have it — and she deserves everything he says.)

“Three Billboards Outside Ebbing, Missouri” provided some of the strongest laughs and some of the most poignant moments of heartbreak of any movie in recent memory. McDormand is sure to score an Oscar nomination for best actress, and the film is deserving of awards consideration in at least a half-dozen other categories as well, from McDonagh’s script and direction to the work by Harrelson and Rockwell.

This is the best movie I’ve seen this year.

★★★★

Fox Searchlight presents a film written and directed by Martin McDonagh. Rated R (for violence, language throughout, and some sexual reference). Running time: 115 minutes. Opens Friday at local theaters.