Gavin O'Connor's "Tumbleweeds" is a tour-de-force of character study, a narrative whose whirlwind plot whips up our interest only to let us down gently in the end.

The opening scene jolts us with a fight between Mary Jo Walker (Janet McTeer) and her latest husband. The vehemence of the quarrel makes us wonder why the movie has only a PG-13 rating. (There's a love scene later that sets us wondering, too.) But we are quickly convinced that Mary Jo is right to get her daughter Ava (Kimberly Brown) out of there - away from this abusive stepfather. The mother and daughter pair sets off in on old Pontiac, leaving North Carolina and heading west to…well, they don't know where.

That's one of the best qualities about the movie, its whimsicality. This spontaneous mother, who does not get upset at things that would make normal mothers livid, carries much of the movie with her eccentricities. We enjoy watching her lack of vexation when the car breaks down - inevitably - on a remote highway somewhere in the dusty middle of the country. When a trucker stops to help, Mary Jo looks over his body as he looks under her hood; meanwhile, Ava, a little girl wise beyond her years, pouts about her mother's flirting. For a long while "Tumbleweeds" acts like a road movie, as the two decide where they might end up on their flight away from the last abusive man. We know, by the way, that Mary Jo has been married four times; what's at least a little puzzling is that she has not been put off starting new relationships almost the hour the old ones are sunk. So when the pair settles in the "Pink Motel" not far from the Pacific Ocean, we think their lives will be calmer.

Big mistake. Her feelers out for a relationship, Mary Jo actually spots the helpful trucker - Jack, played by director Gavin O'Connor - in a bar. They begin a romping romance while Ava is at a sleepover at a new friend's house. The scene in which the girl returns home prematurely reveals a great deal, actually, about Jack. Predictably, this young man ends up not quite meeting the expectations of this impulsive woman…and vice versa. Not even Mary Jo's job is stable, as she is subjected to harassment and whining by her boss boss, a despicable homunculus. Throughout her strife, the anchor in her life is Ava, the girl she named after the glamorous Ava Gardner.

And the chemistry between them is very watchable. We can imagine O'Connor in many scenes just turning McTeer loose, her southern drawl and her lack of inhibitions on full throttle. Several episodes are shot seemingly without script, improvisations that show the strength of the mother-daughter bond. McTeer is at her best when she gets fully into the skin of this promiscuous, rather permissive mom. It's the incongruity of her behavior that engages and mildly shocks us. At the foundation of her character is sadness, though, as we discover when a co-worker friend of Mary Jo's asks her "Why are you going to Arizona? Tell me." again and again. We feel a subtle but powerful impact when the script commences to dig into Mary Jo's psychology. Even Ava deserts her at one point. We begin to ponder, not for the first time, if Mary Jo isn't a terribly bad example, though we probably excuse her stupidity when we remember her affection for the girl.

Kimberly Brown is quite a find. Her accent is also markedly Southern, her manner often more mature than her mom's. Her Ava handles the tension between her mother and various father figures quite well, not wholly succumbing to the conventional pouting and sullenness of children in similar fixes.

Gavin O'Connor acts naturally as Jack the trucker/jerk. He's a little too young for Mary Jo, however, and the script causes him to reveal vital information - like an accident and a job loss -- a little later than he should.

Another male, a co-worker named Dan (Jay O. Sanders), gets along famously with the aspiring actress Ava. Sanders does a modest and strong job as Dan Miller, though the script handicaps him by positioning him as the knight in emotional armor: he's single, having lost his wife in a car accident that he apparently caused. Further, Dan is conveniently available when Mary Jo drops Jack. Sound like a few too many clichés?

A lot of things are nicely realistic about "Tumbleweeds." The characters' lives unfold realistically, albeit a bit dramatically. But larger than life is how most of us like our characters, and Janet McTeer's Mary Jo is a bundle of likable contradictions. "Tumbleweeds" doesn't quite know where to go in its final moments, though unlike many films, it does not wander off and die.