Few movies have ever made as an abrupt a turn as Mexican filmmaker Michel Franco’s “Sundown,” a downbeat melodrama about dissipation, its causes and its consequences.

It features a performance of perfectly poker-faced ennui by Tim Roth, a desperate breakdown and meltdown from Charlotte Gainsbourg and a sobering portrait of the seemingly sleepy but dangerous Mexico beyond the gates of its fortified resorts.

And when it flips, from a Tennessee Williams sojourn, getting lost in booze and languor in a foreign clime into something more climactic, it will make your head spin.

A family is vacationing at a posh Mexican resort. Alice (Gainsbourg) and the drinking-age kids (Albertine Kotting McMillan, Samuel Bottomley) are trying to settle into the siesta atmosphere of Aculpoco.

Neil (Roth) is way ahead of them. He seems lost in his own thoughts, adrift and bored and patiently waiting on that next hand-delivered Dos Equis or marguerita. The one “active” thing he does is notice 20ish Colin’s (Bottomley) budding alcohol problem.

A call makes Alice violate her “no cell phones/no work” policy. “START PACKING.”

Her mother’s going to the hospital and they have to fly home to Britain. They don’t even make it to the airport before the second call comes in.

And they don’t all get on the plane together because Neil, whom she thanked for coming, lost his passport. Weeping and yelling Alice & Co. board. Neil promises to locate his passport or procure a new one and get home ASAP.

But he doesn’t ask to go back to their hotel. “Hotel” is all he tells the driver, and thus he’s deposited at a waterfront dive. “Call me you need anything” is the driver’s broken-English offer to help with “chicas” or what not. That won’t be the last come-on, sales pitch or arm-twist the non-Spanish speaking Neil will face in the coming days.

His blank-faced response is both passive and dismissive. He’s not hearing you, or not listening. He’s got his head in the shade, his feet in the surf and another Dos Equis working. He’s not talking, not even to Alice, whose incessant calls and then “ding ding ding” text messages when he doesn’t answer speak to her frantic state.

“I need your help with the decisions.”

More empty promises of “”I’ll do my best,” more Dos Equis.

Whatever’s going on, Neil won’t engage in it. Whoever among the locals — who can seem predatory, even the fetching bodega owner Berenice (Iazua Larios) who comes on to him — has him “marked,” he’s not concerned.

Even when events go sideways, Neil reacts with a Zen, or zoned-out calm.

Franco, who made “After Lucia” and “Chronic,” does two things to wrong-foot the viewer, little “revelations” I won’t give away. Suffice it to say that the relationships between one and all and the delayed hints at what might drive Neil’s passive response to everything around him and all that happens have rich payoffs and contribute to the film’s sense of misdirection and surprise.

“Sundown” lulls you into thinking it’s one thing, taking on the perfect “Leaving Las Vegas” tone of a sad, solitary man who has checked out. And then it trips you up, even as its anti-hero never breaks his ambling, “whatever” stride.

Roth underplays Neil to the point where we read layers of meaning into his passivity. The actor never lets us see the man concerned or even wholly-engaged in whatever befalls him or those around him. “What is Neil’s game,” we wonder? “What’re his priorities?” Or “What is Neil’s tragic secret?”

Like the master big screen poker player than he is, Roth never ever shows his cards.

And Franco, managing the not-easy feat of making “Mexico away from the resorts” so languidly, liquidly attractive, and yet tragically dangerous, has delivered the first wholly original, instantly memorable movie of 2022.

Rating: R for sexual content, violence, language and some graphic nudity

Cast: Tim Roth, Charlotte Gainsbourg, Iazua Larios and Henry Goodman

Credits: Scripted and directed by Michel Franco. A Bleecker Street release.

Running time: 1:23