Grey skies, a “cool jazz” score and sketchy characters are selling points of “The Mastermind,” a ’70s period piece infused with the “grit” aficionados associate with the films of that benighted decade.

It looks the way fall thrillers, even dryly comic heist pictures like this one, are supposed to look.

The latest from writer-director Kelly Reichardt (“First Cow,” “Meek’s Cutoff” and “Wendy and Lucy”) is a movie that “shows” us rather than “tells” us what’s going on, what’s going wrong and the psychological and intellectual state of the characters.

And what it shows us is a heist that people with no experience in such things pull off and all the things that go can and will go wrong when others pick up on that inexperience.

“I don’t think you thought this thing through,” is something James “J.B.” Mooney hears and hears again in describing the small, suburban Framingham (Massachusetts) Museum of Art robbery he’s masterminded.

It’s not based on a true story, but it was inspired by a couple of rather casual and under “thought through” art-nabbings, possibly the 1990 heist that deprived the Isabella Stewaert Gardner Museum of Boston of works by Rembrandt and Vermeer, and certainly a lower rent 1972 theft from Worcester Art Museum.

Reichardt’s quiet, contemplative style is no more suited to this genre than it was the doomed wagon train Western “Meek’s Cutoff” or the eco-terrorism thriller “Night Moves.” But she uses this caper-gone-wrong plot to serve up a Boomer character study set in protest riven 1970, as our title character finds himself making an odyssey through the American counterculture as he tries to flee the country.

Josh O’Connor from “Emma.” and “Challengers” is our title character, a seemingly aimless “art school dropout” with a failing cabinetry business, an obliging working wife (Alana Haim) and two tween boys, at least one of whom is what we might now refer to as “on the spectrum.” The kid won’t shut up about his obsessions, and even folds and tosses paper airplanes in the local museum, where Mom and Dad end up taking them, age appropriate or not.

The kid is useful to 30something J.B., a born “distraction” and a reliable indicator of which guards nap on the job and how much attention the average patron would pay to something noisy and out of the ordinary. In the parlance of the trade, J.B is “casing the joint.”

But he’s there so often he has to assemble some locals (Eli Gelb, Javion Allen and Cole Doman) to do the deed for him — four abstract paintings by Arthur Dove, to be grabbed and stuffed into sacks from a little-visited gallery within the (fictional) Framingham museum.

As J.B. has pocketed a few miniature collectibles from the collection, with wife Terry as an accomplice, and she sews the sacks the paintings are to be stuffed in, we assume she’s in on it. But maybe not.

The heist hasn’t even happened when we start to count the loose ends J.B. hasn’t planned for. When the driver quits after getting paid to steal the getaway car, everything sort of goes downhill, from a cop taking his lunchbox out to take a break in the rear entrance, to a high school driver blocking the stolen getaway car, to the one guard who puts up a fight.

J.B.’s father (Bill Camp) is the first to cluck “didn’t think this through” while reading the newspaper account of the robbery at family Sunday dinner. Mother (Hope Davis) doesn’t realize the loan she slipped to her son for a “business opportunity” just financed the heist.

Reichardt takes her time setting up this slow-motion trainwreck and keeps her cards close to her vest in terms of character details that underscore just how “wrong” this whole thing goes. She spares us the melodramatics and just lets things happen and the consequences be accepted in ways no conventional thriller would.

She showcases the anti-war protests that were the signature of a generation, and then has J.B. cynically exploit them and his old college pals (Michael Angarano and Gabby Hoffman) and “the Movement” to attempt his getaway.

As a writer, director and editor, Reichardt pays a lot of attention to pacing, the more deliberate the better. We watch painstaking efforts to hide the paintings, J.B.’s meticulous attempt to swap pictures from a passport he’s come by, and find outselves frustrated by the passivity of it all.

Pacing that slow is not to every taste, and in the case of this dip into the heist picture genre, we see the narrative’s momentum slowed and stakes lowered by her “patience.” But Rob Mazurek’s glorious muted horns, period-jazz (Think Lalo Schifrin and “Bullitt”) score contrasts with her dialogue-free, overly-deliberate “detail” scenes and lifts the picture and gives it momentum.

And every time we allow ourselves to walk in J.B.’s dirty white sneakers, identifying with the man on the lam, pondering how he’ll get away with it, somebody else comes along — maybe a mobster (Matthew Maher of “Air”) — to remind him and us that maybe “you didn’t think this thing through.” Because he didn’t.

Rating: R, profanity, a hint of violence

Cast: Josh O’Connor, Alana Haim, Eli Gelb, Javion Allen, Gabby Hoffman, Michael Angarano, Matthew Maher, Hope Davis and Bill Camp

Credits: Scripted, directed and edited by Kelly Reichardt. A Mubi release.

Running time: 1:50