Leave it to Kelly Reichardt to make a ‘70s movie that looks and feels like a lost ‘70s movie, from its scruffy visual aesthetic to its muted colors, its patient character observation and unhurried pacing to its unstinting investment in an underdog protagonist whose careful planning results in a coup that soon goes south. Josh O’Connor’s rumpled appeal makes him an ideal fit for the title role in The Mastermind, a minor-key heist caper that spends as much or more time on the aftermath of the crime, when it morphs gracefully into another of the director’s singular character studies of struggling Americans.

The film is set in Massachusetts circa 1970, two decades before the infamous art theft at Boston’s Isabella Stewart Gardner Museum, whose walls still conserve the empty spaces where stolen paintings by artists including Vermeer, Rembrandt, Manet and Degas once hung. It seems like quintessential Reichardt that James Blaine Mooney (O’Connor) is not going after the Old Masters or anything even close in value. Instead, he targets four paintings by American modernist Arthur Dove, one of the country’s pioneering abstract painters — influential but back then not in high demand.

Reichardt takes her first solo writing credit on this feature, which nonetheless has echoes of two films penned with frequent screenwriting collaborator Jonathan Raymond. It has shades of the meticulous planning of the eco-activists who blow up a hydroelectric dam in Night Moves and continues the vein of subtle humor that made the microcosmic art world view in Showing Up so captivating.

The opening sequence follows J.B. as he walks from room to room, studying both the art and the snoozing guard in a fictional museum in Framingham. (Stand-in for the exteriors is the I.M. Pei-designed Cleo Rogers Memorial Library with its massive Henry Moore bronze out front, memorably showcased in the beautiful Kogonada film, Columbus.) One half of a pair of young twins prattles on incessantly about some sci-fi arcana while the boy’s bored-looking mother and his quieter brother tune him out.

Only once J.B. has opened a display cabinet to pilfer a small artifact and they head for the exit does it become clear that the woman is his wife Terri (Alana Haim) and the kids are his sons, Carl and Tommy (Sterling and Jasper Thompson). Terri appears to be an accomplice while the boys serve as decoys, which initially calls to mind stories of families in petty crime cahoots like Hirokazu Kore-eda’s masterpiece, Shoplifters. But that proves to be a bit of crafty misdirection.

When James moves beyond small trial runs and prepares to lift the Dove paintings, Terri seems to want to know as little as possible. James puts together a team of three, Guy (Eli Gelb, one of the discoveries of Broadway hit Stereophonics), Larry (Cole Doman) and Ronnie Gibson (Javion Allen), assuring them they will be in and out in eight minutes. James explains that he can’t be there while the heist is going down because his face is now too well known to museum staff.

But when Larry bails as driver, J.B. has to fill that role, and although they do get the paintings out, things don’t entirely go according to plan thanks to Ronnie, who pulls a gun on an art student and gets into a scuffle with a security guard at the exit. Several scenes later, after Ronnie has caused further trouble, J.B. gets a too-late lesson in the mocking words of a savvier thief (Matthew Maher): “Never work with drug addicts, dealers or wild cards.”

Once news of the daring daylight art heist breaks, J.B.’s father, Bill (Bill Camp), a local judge, also has thoughts that might have been more useful before the event: “It seems inconceivable that these abstract paintings would be worth the trouble.” One of the great contemporary character actors, Camp dials up the pomposity as Judge Mooney muses about the dark market before conceding, “These things are outside my realm of experience.”

Bill’s criticism of unemployed James for not making something of his carpentry skills like a small business owner with whom he was at school seem a significant factor in J.B’s decision to try making money the easy way. Dishonestly. His mother, Sarah (Hope Davis, sublime), is more indulgent with him, though when he hits her up for a sizeable loan on top of money he already owes her, she insists on a scheduled repayment plan.

While Reichardt never pushes for comedy, these fusty parental exchanges are often very funny, as are J.B.’s bad-parenting episodes with the boys.

Period production and costume design (by Anthony Gasparro and Amy Roth, respectively) are instantly evocative of the era, while being careful never to distract with conspicuous kitsch. But some relics of the ‘70s inevitably get laughs — the crank-handle rear window that gives Guy trouble while he’s rushing to load the paintings into the back of a stolen station wagon; the forgotten marketing gimmick of L’eggs Pantyhose, sold in plastic egg-shaped packaging, which J.B. provides to his crew to wear as masks.

Reichardt finds infectious fascination in some of the more mundane elements of the crime, such as James applying his carpentry know-how to build a tailor-made storage crate for the paintings. That crate then yields physical comedy when he crawls up a ladder to hide it in a hayloft while a pig snorts away in the background, snarfling for food and paying J.B. no attention.

Playing a character who might easily be an American cousin to his sad-sack grave robber in La Chimera, O’Connor deftly balances those comic moments with a slow build of melancholy and regret — “I didn’t really think it through,” he says morosely — as J.B.’s get-rich-quick scheme slips out of his reach.

Haim, the singer who became a breakout screen star in Paul Thomas Anderson’s Licorice Pizza, has less to work with as Terri. But she says a lot with her eyes about the character’s internal battle between forbearance and walking away to protect herself and the boys from James’ wreckage. There seems genuine regret on both sides when James loses Terri as an ally.

The economy of Haim’s performance is very much in keeping with Reichardt’s less-is-more policy with her actors, which applies to the incisive casting even of the smallest roles, with faces that look right at home in the era.

There’s an interlude both lovely and sad in which James is still at large despite his face being splashed across newspapers. O’Connor strikes poignant notes when J.B. fools himself into thinking he’s safe while laying low at the farm of his old friend Fred (indispensable Reichardt regular John Magaro) and his wife Maude (Gaby Hoffmann), who is convinced James is using their old college art professor as his fence. Fred appears quite excited to have a wanted felon in their midst, Maude considerably less so, which hastens J.B.’s departure.

Throughout the film, newspaper headlines and snippets of TV contextualize the story against the backdrop of anti-Vietnam demonstrations, colleges retaliating to student campus protests and aggressive policing, along with glimpses of Richard Nixon’s crooked grin. While Reichardt is careful not to hammer this element too loudly, it’s impossible to miss the parallels with today’s political landscape.

James’ attempted flight to Canada hits a snag during one of those street protests, and the final shot of him, boxed into a small part of the frame, is crushing.

Longtime DP collaborator Christopher Blauvelt, who also shot Meek’s Cutoff, Night Moves, Certain Women, First Cow and Showing Up for Reichardt, remains a matchless fit for the director’s naturalistic minimalism, ensuring that even rows of trees in blazing fall colors are never overly pretty.

As she did with Night Moves, Reichardt has made a genre picture that peels away all the usual tropes to focus on character, on human failings and on the reality that even someone from a comfortable middle-class background can be worn down by struggle and reach for unwise solutions.

The only major departure for Reichardt is the highly effective use of a score by jazz musician Rob Mazurek. The cool, but also nervy riffs of percussion, bass, brass and drums sound like the work of a beatnik dive bar ensemble winding down at the end of a long set, providing the perfect complement to a decelerated movie that runs on understatement.