Two 10-year-old runaways pick the wrong unmanned police cruiser to take for a joyride in "Cop Car," a stripped-down, steadily simmering thriller from writer-director Jon Watts (whose feature debut, the intensely creepy "Clown," began as a fake trailer that turned into a viral sensation). Harrison (Hays Wellford) and Travis (James Freedson-Jackson) are making their way by foot across the rural Colorado countryside when they come upon a patrol car. Their childish games of running up to touch the vehicle before hiding again in the underbrush escalate quickly once it looks like there is no officer in sight. When they find the car keys hidden in the sun visor, they cannot stop themselves from heading out for a wild drive. What they do not know is that the car belongs to crooked Sheriff Mitch Kretzer (Kevin Bacon), and the body of one of his recent victims is still in the trunk.

"Cop Car" reminds of a slightly simplified "Blood Simple"/"Fargo"/"No Country for Old Men"-style Coen Brothers movie that happens to star a pair of prepubescent kids. The screenplay by Watts and Christopher Ford aims for mood and quietly mounting tension over corkscrew plotting, its straightforwardness an attribute until the third act misses the opportunity to dig deeper and find purpose beneath its thin surface veneer. If it is never clear what the ultimate purpose of the picture is rather than, perhaps, as a cautionary fable about the dangers of breaking the law in an already unforgiving adult world, Watt's directorial deftness is never in doubt. There isn't a wasted frame to be hand, grabbing one's attention from the start as young Harrison and Travis' misguided juvenile decisions move ever closer to colliding with Kevin Bacon's (2015's "Black Mass") desperate, menacing Sheriff Kretzer. "Cop Car" is involving enough for the viewer to wish the film would better explore its narrow handful of characters rather than as pawns in an inevitable violent showdown. Nevertheless, as an envoy for beautifully lonesome wide open spaces and pure slow-burn suspense, it rattles along with undeniable know-how.