Trieste Kelly Dunn stars in “Cold Weather” as one of a trio of twentysomethings who are caught up in a mystery.

This may sound like a contradiction in terms, but “Cold Weather” is a mumblecore movie with a purpose.

Sure, its twentysomething characters sit around and talk about nothing in particular. They're unmotivated to work at jobs that don't exactly matter to them. And writer-director Aaron Katz, a founder of this film genre, isn't shy about lingering on a shot, such as when his characters are doing nothing more exciting than eating lunch at a park bench along the Oregon coast.

But shots like that set the tone and subtly lure us in. And so when the tension does build — and really, surprisingly, it does — it sneaks up on us.

Much of the film's allure comes from the naturalism of its settings, performances and dialogue.

Cris Lankenau stars as Doug, a college dropout who has moved back to Portland to live with his older sister, Gail (Trieste Kelly Dunn), in a nondescript apartment. They hang out, drink beer and play cards; Eventually he takes a minimum-wage job at an ice factory, where he befriends his co-worker, Carlos (Raul Castillo), an aspiring DJ.

Are you hooked yet?

Things do pick up, though, when Doug's ex-girlfriend, Rachel (Robyn Rikoon), shows up on a vague business trip from Chicago — and then she's gone just as quickly as she came. Having studied forensic science during his brief stint in college, Doug is still fascinated by Sherlock Holmes novels. He ends up playing amateur detective and, with Carlos and Gail's help, they try and piece together what they assume are bits of evidence in hopes of tracking Rachel down.

Katz fills “Cold Weather” with many of the usual conventions you'd expect in a thriller or mystery — a code to crack, a stakeout, a car chase — but he uses them more as opportunities to develop his characters. “Cold Weather” never turns too self-serious, but Doug always takes his mission seriously, and the cool, cloudy blues of Andrew Reed's cinematography add to the gloomy, moody atmosphere.

The fact that the stakes end up being ridiculously low doesn't matter. That's just one more appealing element of the film's unconventional intrigue.

TITLE: “Cold Weather”CAST: Chris Lankenau, Trieste Kelly Dunn, Raul Castillo, Robyin RikoonDIRECTOR: Aaron KatzRATED: Not rated but contains language and nudityGRADE: * * * * (out of 5)