Fred Hechinger, Clark Gregg, Parker Posey and the late Richard Roundtree also star in Josh Margolin’s account of an elderly widow going after the scammer who duped her.

Josh Margolin drew inspiration from an attempted scam in which a caller posing as him almost succeeded in conning money out of his grandmother, whose name provides the title of the writer-director’s entertaining feature debut, Thelma. Unlike the real-life situation, the 93-year-old character played with warmth, humor and formidable tenacity by the marvelous June Squibb falls for the ruse, getting bilked out of $10,000 before she even has time to think about it. But Thelma Post is not the type of woman to shrug off the unpleasant incident, no matter how much her family underestimates her.

One of the key assets of the appealing ensemble, alongside Squibb, is Fred Hechinger, who was so terrific on season one of The White Lotus. (Hey, Mike White, it’s not too late for a spinoff series about Quinn’s life with the Hawaiian canoe paddlers and his folks’ efforts to get him back home.) Hechinger plays Thelma’s adoring grandson Daniel, introduced patiently helping her wade through a thicket of emails in an opening scene that establishes the limits of her computer literacy.

But even though Thelma is wobbly on her feet and lives in fear of a fall, she’s far from helpless. One minute she’s doing needlepoint while watching TV, admiring the athletic form of Tom Cruise’s trademark sprint in Mission: Impossible — Fallout, the next she’s zipping out of the assisted living facility that’s home to her late husband’s friend Ben (Richard Roundtree), attempting to abscond with his mobility scooter. Ben catches her just in time, and since he’s too much of a gentleman to let her face danger alone, he insists on accompanying her across sprawling Los Angeles to the post-box address where she mailed the money.

Much of the movie’s charm is in the buddy-comedy element of Thelma and Ben’s misadventures on the road. Thelma is stubbornly proud to remain independent; she misses her husband but at the same time is enjoying the experience of being alone for the first time in her life. Ben, by contrast, readily acknowledges that they are both old and diminished: “We’re not what we were.” He’s grateful for the security afforded him by the retirement home and the activities provided there to ward off loneliness.

Margolin’s writing gets a little self-consciously cute at times, but more often, it’s genuinely touching, fortified by the relaxed rapport between Squibb and Roundtree and the good-humored bickering between their characters. Thelma’s prickliness and the mischievous glint in her eyes are nicely offset by Ben’s kindness, particularly when she suffers a physical setback.

Despite providing Thelma with a gun and her own version of spy gadgetry in a hearing-aid phone app, Margolin keeps the Ethan Hunt parallels more or less within the bounds of plausibility. Even a cheeky riff on the familiar trope of action heroes sauntering away from an explosion feels more grounded in character than genre. The film makes Thelma’s natural gumption her strongest weapon, allowing her to stay one step ahead of the panicked Gail and Alan, to face trouble with a cool head and to call on the assistance of Daniel only when necessary.

The mutual confidence boost exchanged between grandmother and grandson is nicely underplayed by Squibb and Eichinger, giving the modest movie real heart. Thelma is sweet and poignant, sentimental without getting sappy. Just the fact that it gives Squibb the first leading screen role of her seven-decade career makes it a satisfying watch.