Most mainstream films are like pulp novellas, full of trite turns and a dramatically shallow ending. They’re easy to read: find the hero, spot the villain, evaluate accordingly. But how do you review a haiku? All you can say is whether it works or not. Sometimes its modest elegance is just perfect, and you know it when you see it.

Ken Lonergan’s new film “You Can Count on Me” is like a terse haiku. A treatise on human staples — life, family, love, loss — it nevertheless strikes one as so simple, honest and pithy that one is reintroduced to emotion.

Audiences are getting used to brainy indies. We can spot their clever scripts and unknown actors from miles away. But Lonergan’s film is different. Its impact lies not in novelty, but in the soft murmur that saddens one for days after seeing the film. Its haunting is almost comforting.

We begin with a trauma. The parents of a young girl and boy are killed in a (perfectly understated) car accident. This loss frames the film’s narrative, just as it frames the lives of the two children.

Now grown up, Sammy Prescott (played by Laura Linney, who has been nominated for a well-deserved Oscar) is a single mother. She lives in her parents’ old home, having never left her small upstate New York town. Her son is Rudy Jr. (Rory Culkin), a brainy yet appropriately petulant eight-year-old boy.

Sammy is a lending officer for a bank whose new manager, Brian Everett (Matthew Broderick), is anality personified. Broderick has lately been the most well-known name in several offbeat films (think “Election”), and his talent as a character actor may very well surpass his legacy as Ferris Bueller.

If audiences cheered during Helen Hunt’s HMO scene in “As Good as it Gets,” then I expect veritable riots when Brian refuses to allow Sammy 15 minutes a day to pick up her son from school. Though not the focus of the film, the trials of a single mother, whether black from the city or white from the boondocks, are very realistically depicted here.

The adult son is Terry Prescott (Mark Ruffalo), a drifter and small-time ne’er-do-well. He gets thrown in jail for bar fights and occasionally smokes pot, but is kind and has a conscience. While most reviewers have raved about Ruffalo, they tend to emphasize aesthetics over craft. Admittedly, he is gorgeous, but his alternation between tenderness and coarseness is not easily achievable and I’m not surprised that he hails from live theater.

Terry visits Sammy and Rudy to borrow some money. He stays longer than expected, but leaves in the end. Although the modest narrative is a pleasure to discover as it unfolds, you probably guessed that in many ways Terry becomes a father-surrogate for Rudy Jr.; Sammy learns to relax a little, and everyone involved grows up. This rendering of events captures the general trajectory of the film, but rest assured that its delivery is far more intricate.

The greatest strength of “You Can Count on Me” is its script and its characters. You won’t find a single human cut-out in this film, and once you see it, you’ll realize how shallow most film scripts are. Sammy could have been an uptight, religious, small town, small-minded single mother. Rather, she is strong and vulnerable, funny and mischievous, sexual and regretful. She loses her temper at her son and her brother, but loves both so deeply we forgive her instantly.

Terry could have been the reckless, aimless brother, returning as the Prodigal Son. But his affection for Rudy Jr. is genuine and his depth of conscience is palpable. Brian could have been an evil bureaucratic manager, but his teasing affection for Sammy and love-starved marriage make him human. Rudy Jr. is not Little Man Tate or our saccharin prodigy from “The Sixth Sense” — he is appropriately obnoxious, annoying, irrational and lovable (as is every real eight-year-old boy).

Though the film is not a meditation on generations, Sammy and Terry are ostensible members of Generation X, and they do live within a post-Vietnam, post-meaning, post-modern age. And though it is not a meditation on religion, Sammy’s muted faith and Terry’s aimless nihilism are contrasted with a secular appreciation for meaning and purpose in one’s life. One of the most likable and consistently reasonable characters is Father Ron (played by Lonergan), whose belief in service and community indicates a refreshing, unpretentious maturity.

What a cliche to say that life is complicated. What a lie to say that it isn’t. Everything about Lonergan’s film suggests its own realism, and it’s a realism you won’t see in mainstream films: silences, pauses, awkward laughter, imperfections. The characters dress, talk, act, cry and feel like real people. Our need for voyeurism is so adequately met here that one almost feels dirty for watching the private moments of our cinematic neighbors.

Whenever you watch an independent film, do a thought experiment over coffee afterwards: What Would Hollywood Do? After the car accident, when the sheriff comes to tell the children about their parents’ death, we would have seen every excruciating overplayed minute of the revelation. Here, the film cuts. At the end, when Sammy and Terry reminisce about their childhood, we would have trite lines that no human being would actually ever say to one another — “Remember when mom used to tell us to love each other no matter what. Well I do! I really do love you!!” Ugh. Here, we have a tearful embrace and appropriately veiled memories.

Lonergan doesn’t satisfy our desire for a safe, smooth resolution. He challenges his audience to feel alone and vulnerable along with his characters. In most films, the audience gets to be superior to the film’s subject. Here we’re on uncomfortably equal footing.

In the end, “You can Count on Me” is so wrenching precisely because of the irony of its title. I’m not positive that these characters can count on each other to be anything other than human, frail and fallible. I am positive that the film concludes with Terry leaving against Sammy’s will. And this loss is the inevitable loss of family, loves, lives.

If death is at the center of human drives and longings, it is certainly at the core of this film. Loss is the one human certainty. Well, loss and love.