After eight years of solid supporting work, and an especially memorable co-starring turn opposite Casey Affleck in the 2007 "Gone Baby Gone," Michelle Monaghan has finally been put at the center of a movie, where she belongs. She doesn't leap at the role — the picture is too modestly scaled to withstand a full-glow starburst — but she inhabits it completely.

In "Trucker," Monaghan plays Diane Ford, who makes her way in life as a long-distance truck driver. ("That's a weird job for a woman," someone tells her. "Why?" she asks.) Diane is serious about her work; it's a good fit for her no-strings lifestyle. Her sexual encounters are strictly one-nighters, over as quickly as she can hurry back into her jeans. Her house in a shabby California desert town is almost paid off, and between jobs hauling freight, she spends her only quality time with a drinking buddy called Runner (Nathan Fillion). It's a bare-bones life, but that's the way she wants it; she doesn't need company.

Diane was once married to a good guy named Len (Benjamin Bratt). After having a baby, though, she got antsy and scared, and took off in the night with no word of explanation. Now, 10 years later, she learns that Len is dying of cancer in Los Angeles. His current wife (Joey Lauren Adams), implores Diane to help out for a few weeks by taking care of 11-year-old Peter (Jimmy Bennett), the son she's never known. The movie is naturally focused on the initially prickly interaction between Diane and Peter. Dropped off at her house, the boy's fierce resentment of this disruption in their non-relationship matches Diane's own dislike of his intrusion into her self-contained life.

A lesser film might have outfitted these two characters with a complement of cutely barbed one-liners to illustrate the grudging affection they're afraid to acknowledge. It's a credit to first-time director James Mottern, who also wrote the script, that the picture doesn't do this. Diane and Peter really dislike each other, and we see why. (She can only bring herself to call him "kid"; he occasionally addresses her as "bitch.") Being a mother is outside her realm of comprehension, and Peter knows it. Whenever it seems like a thaw might set in — a tentative bonding over baseball, for example — one or the other of them screws it up.

Nathan Fillion suppresses his familiar jaunty appeal here to give one of his most affecting performances. His Runner is half a drunk locked into a dead marriage; Diane is his only friend, and he longs for something deeper with her but is afraid she'll bolt if he says so. Bennett is especially impressive as a kid who's built up emotional walls so high he can barely see over them. And Bratt has rarely been better than he is in this picture; with very little screen time, he vibrantly conveys the dying Len's enduring bewilderment about his wife's long-ago desertion — a puzzlement he'll now take to his grave.

Monaghan rules the movie, though, holding us close to a woman with no particularly likable qualities. We can see that Diane has made some harsh choices in her life and is now beginning to realize that she no longer remembers why. Monaghan conveys the character's boxed-in emotional conflicts through a flickering wariness and constricted body language. Diane seems hopeless, but the actress keeps us holding onto hope for her anyway.

The movie is built entirely on character detail, and it's carefully unemphatic. It never goes exactly where you might expect it to, and it resists sugary resolution. After a burst of unexpected action, the picture ends quietly, with a single softly spoken word. As with the rest of the film, it's just right.

Don't miss Kurt Loder's reviews of "Paranormal Activity" and "An Education," both new in theaters this week.

Check out everything we've got on "Trucker."

For breaking news, celebrity columns, humor and more — updated around the clock — visit MTVMoviesBlog.com.