“Post Tenebras Lux” opens in a dreamy Arcadia, a pasture where horses, dogs and cows graze or run around a joyful little girl taking halting steps. It’s only when dusk is slowly turning to darkness and the girl calls for her absent mother that you realize the film’s mythic beginning could be curdling into a nightmare.

But we’ll never know, exactly. Mexican filmmaker Carlos Reygadas’ jagged, broken-logic drama lurches from fantasy to earthbound complications, though it is often hard to tell one from the other.

As a rule, that’s not a problem in the hands of a good artist, and Reygadas (“Japón”) is good. But there is something willfully and stubbornly oblique about “Post Tenebras Lux” (ironically, the Latin title means “after darkness, light”) that eventually replaces a viewer’s goodwill with impatience.

Scattered like marbles on cold steel, the film’s haphazard scenes about an upscale family’s move from a city to a lush farm region suggest story continuity and tension. In fact, Reygadas formally eschews both.

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Juan (Adolfo Jiménez Castro) and Natalia (Nathalia Acevedo) are a troubled couple with two small children (Reygadas’ own kids, Rut and Eleazar). We see them at home, on a beach, at a big family gathering and in community settings where they don’t quite fit in with laborers and small farmers.

We also see Juan attack a family dog in a fit of rage, hector a weary Natalia over trifles and disrobe her before a group of strangers at a sex club. Reygadas presents all of this as desultory slices of heightened reality, dismissing narrative flow. The result is unassembled puzzle pieces of a glimpse into subconscious frenzy.

That can be very effective in a good film. But in “Post Tenebras Lux” — with its out-of-nowhere images of a rugby match and other distractions — the challenge outweighs the reward.

Tom Keogh: tomwkeogh@gmail.com