Kent Jones, who wrote and directed Diane, puts us in the presence of death before he even lets there be light: He begins with a black screen and the beep of a vital-signs monitor connected to a woman in the last stages of cervical cancer. Not a hopeful sound. But the first shot isn’t of the dying woman, Donna (Deirdre O’Connell). It’s of her cousin, Diane (Mary Kay Place), asleep in a chair in the hospital room. And it’s Donna — emaciated, attached to machines — looking on with concern. The image of the worried terminal patient, which is disconcerting, is a sign of what’s to come. Diane is saintlike in her devotion to others; she drives from the hospital to the homes of elderly friends to a soup kitchen (to serve macaroni and cheese) to the grubby apartment of her son, Brian (Jake Lacy), who has plainly fallen back into drug addiction, to the hospital again. She dutifully runs herself down, day after day, as if expiating her sins. But it turns out there’s a cost to being so present, so much in the world: You can’t escape the knowledge that you’re here today, gone tomorrow. At the end of that first scene, Diane wakes with a start and apologizes to her cousin for being absent for a moment, because absence is not what she does.

The opening of Diane is simple but packed, like the movie: The more mundane the details, the more redolent it is of time going by too fast. Someone I know called it the most depressing film she’d ever seen. I found it one of the most exhilarating, but I admit that the exhilaration is hard-won and slightly perverse. You have to accept the bleak premise as something to move through and beyond, as Diane will finally, in a way, though not happily. (You would have to be a saint — or demented — to be happy about it.) The elderly — Diane’s aunts, the parents of friends — gather around kitchen tables, laughing and reminiscing as their bodies fail, and you have to take what pleasure you can. Invariably, they ask about Brian, and she says he’s okay and then, a beat later, not okay, and they tell her that she has done all she can do, that he’ll have to save himself now. Diane listens and then drives to her son’s apartment with groceries or clean, folded laundry and tries to jar him from his stupor. She wants him to go back to rehab, and he snarls at her to leave him be, which she never will. Driving is the movie’s central motif. In between the scenes, shots through a windshield pass rural landscapes in all seasons, often with soft, haunting music by Jeremiah Bornfield. We don’t see Diane at the wheel — it’s her point of view, or Jones’s. Sometimes the driving shots separate years. I have a little metaphor horn that goes off in my head when I see those kinds of shots, but like all of Jones’s metaphors, it’s grounded, tactile. I know that windshield. I know those roads. I know the aloneness between someplace and someplace else.

I’ve rarely felt, as I did at Diane, that the director was reading my mind and always several steps ahead. Halfway through, I began to dread that the film would be one grimly realistic scene after another with no glimmer of light at the end of the tunnel, no hint of ­transcendence — at which point, Diane suddenly stops by the side of a road and trudges up a snowy embankment into the woods, peering at the sky through branches, desperate for a breather from time itself. Shortly after that comes a cut to Diane in an Evangelical church amid people rocking and babbling in tongues, and I feared she’d find that escape in blind worship. But no, not to worry; it’s another character who’s born again. For Diane, that sort of release would be too easy, too selfish. She has sentenced herself to the here and now.

In his first feature, Jones — who directs the New York Film Festival and has made documentaries on Val Lewton and the Hitchcock-Truffaut interviews — hits a few notes overly hard. A shot of Diane moving in slow motion when she thinks Brian has OD’d is too self-conscious. When a gentle, kindly regular at the soup kitchen, Tom (Charles Weldon), tells her that taking food from her makes him feel sanctified, it’s too on the nose. The geography is puzzling; Diane is set in Massachusetts, but the landscape and accents seem like upstate New York. There are some easy laughs at the expense of Brian’s born-again wife, Tally (Celia Keenan-Bolger), although fundamentalists who aggressively proselytize do tend to caricature themselves.

Every other detail is right, sometimes startlingly right. When Donna dies, her mother, Mary (Estelle Parsons), rushes from the bedside to a crying Diane and says, “She loved you. She loved you.” That moment isn’t set up, but it seems natural for Mary, who knows she won’t live much longer, to think about the one who has to carry on. Mary’s throwaway remark about a friend of hers — she’s “dumb as a box of rocks, but she’s always been a good friend to me” — is just what an old woman whose contemporaries have mostly died would say about someone she now relies on for company. There must be a hundred lines like that in Diane, casual but resonant (“How’s Mario’s shoulder?”) and so authentic that you know Jones has breathed the same air as his characters. (And he has shared the same light: Without being too dark, Wyatt Garfield’s cinematography evokes a life of standing lamps that only partially illuminate the space.) No one in the marvelous cast—Lacy, Parsons, Joyce Van Patten, Glynnis O’Connor, that spitfire Andrea Martin, and so many others — seems to be acting. Just being.

I don’t know how to do justice to Mary Kay Place. It’s not a self-effacing performance; it’s a portrait of a person who’s laboring to be self-effacing, to strip away all the inessentials, her vulnerability slipping out in spite of her best efforts. Diane is busy even in private, making to-do lists and then turning to a journal as her friends die one by one. Gradually, we learn that she has sinned in her own eyes but also that the sin was her truest moment of freedom from the heaviness of her life. What she did comes back to her in dreams that are spooky, from another world. Her regrets and her longings merge.

Early in the film, Diane tells her hopeless, addicted son, “You’re not alone,” and her friends say the same thing to her in the rare moments when she loses control. But as the people around her die, she feels her aloneness more vividly. We are very much alone, the film suggests — it’s why we need so much reassurance. But trying to forestall our loneliness is what connects us. That’s not a depressing conclusion — it’s the beginning of a design for living. In its mundane way, Diane gives you a glimpse of the sublime.