Promising and disappointing all at once, Jon Watts' backroads thriller Cop Car heralds the arrival of a significant director, one adept not just at the usual action and suspense but also at the fleet, affecting depiction of lives as they're actually lived.

In the opening scenes, the camera glides alongside elementary-school boys as they tromp through a field in one of those empty stretches of Colorado where the plains lift toward the Rockies. The kids dare one another to say out loud the most unspeakable words they can think of; then they slip through one of the barbed-wire fences that stitch their world from horizon to horizon. One boy's uncertain in his baby fat, fumbling with the strands. The camera, though, slides right through, its movement smooth but not flashy — Watts stirs the sense that we're picking along the landscape, too. That immersion in the kids' vague adventure peaks with the film's inspired first revelation: In the woods edging that field, the boys find a police cruiser, far off the road. A door is unlocked, the keys are on the seat and there's nobody around.

So they joyride, and the film — for its first 20 minutes — lets us do so, too. Watts, who co-wrote with Christopher Ford, is sensitive to his small-fry heroes' fears and enthusiasms and playful illogic: Often, the boys seem to be in a space between play and life, and as the plot kicks in, and they hit the road, they look to one another for clues as to whether they're in real danger or pretend.

Movies must grind on for 90 minutes, though, so the play must become something more. That's where the disappointment comes in. After the joyous, specific evocation of the lives, inner and outer, of these small-town kids, Cop Car invests itself in indie-thriller generalities: The sumbitch sheriff up to no good, a shootout along a stretch of two-lane blacktop, a pitilessness about killing off incidental characters that might feel to the filmmakers like tough-mindedness but plays as grimdumb cruelty. Watts and his crew capably stage and cut all this, but the material is expressive of nothing except itself: The film soars early as a fantasy steeped in life and crashes into a drag of a crime drama, one ripped from the movies rather than anyone's idea of small-town Colorado.

Kevin Bacon plays the sheriff, all mean leanness and a twitchy mustache. He's amusingly stiff and beleaguered in the film's first half, especially in the surprise flashback where we learn why he's parked his car in the woods — and what bad news it is for him to find it gone. I say “surprise flashback” because Cop Car, in its commanding first reels, trusts us to work out the chronology — at one point, it's not the kids driving anymore, it's the sheriff, an hour or so before, and you might not know at first. Bacon's biggest laugh comes the first time he speaks: The sheriff is desperate, up to no good and all alone in the middle of nowhere, but when he radios dispatch with a cooked-up cover story, he's all peaches-and-cream politeness. But after that, the sheriff offers little that's new or surprising — and Cop Car gives this character a lead's worth of screentime.

Eventually the film conforms to too-established generic principles. Bacon goes pure villain, the kids learn a lesson, and only scattered moments of the third act approach the promise of the first: The last 90 seconds or so are excellent, and there are some fine, nervy moments when the kids terrify by being naive with guns — but the surprise is all gone.

COP CAR | Directed by Jon Watts | Written by Watts and Christopher D. Ford | Focus World

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