“Synchronic” is a time-travel thriller with high stakes and genuine pathos. And all of that is conveyed in a gripping performance by Anthony Mackie, who takes this too-rare leading man role to its emotional limits.

The set-up is screwy and simple, the setting riveting and the key ingredients are not necessarily what the picture is about, but pivotal to its power.

Mackie and Jamie Dornan co-star as paramedic pals working their way through a New Orleans over-dose epidemic.

Dennis (Dornan) is a family man, with an infant and a teen daughter as compensation for a marriage (Katie Aselton) he’s constantly complaining about.

Steve? He’s a loner and a Lothario, waking up in a lot of different beds. Tara (Asleton) explains Steve’s “situation” to the baby she brings to a group picnic.

“Look babe,” she coos, “Uncle Steve’s sitting ALL the way over here because he slept with all Mommy’s friends before they married those men over THERE.”

Work nights are nightmarish — junkies and corpses and little wrappers of this “designer drug,” Synchronic lying near many of the victims.

Steve finds out what the drug does long about the time he finds out he’s got a health issue, and just as they stumble across an OD at a party where Dennis’s daughter Brianna (Ally Ioannides) was last seen.

That pill — and this is the “silly” part — sends you on a trip, not in space but in time. Pop a pill and you might wind up on a plantation, in swampland before it was filled in to make modern New Orleans, in the Ice Age and contending with a conquistador, voodoo cult, Confederate soldier or hunter-gatherer.

Co-directors Justin Benson and Aaron Moorhead make great gritty use of the ruined parts of Louisiana, and the time travel bits — often at night — are atmospheric and spooky.

A hallucinatory and wordless prologue sets up the “trippy” nature of the film, and the principal effect — bodies perforated and dissolving from place to place. The shift to the “real” world of EMTs killing time between morbid calls by hitting golf balls into whatever vacant lot they’re parked next to is abrupt means we’ll go a long while before having any idea what that introduction was about.

The story’s stakes come from the ticking clock of Steve’s illness, the limited supply of pills and the chance that “the wild cards of fate” have sent Brianna somewhere in time, with only “armchair physicist (Hah!)” Steve to rescue her.

Mackie makes the quibbles with the “logic” of it all fade into the background with a performance begins brusque and bluff, and softens as he starts to experience the precious brevity of life and the wonders — and limitations — of this dangerous drug.

“Synchronic” scores a few points for its novel choice of “explanation” for its form of time travel, and a lot more for casting the right time traveler to say “Man, f— ‘Back to the Future!’ The past was HELL.”

MPAA Rating: R for drug content and language throughout, and for some violent/bloody images

Cast: Anthony Mackie, Jamie Dornan, Katie Aselton and Ally Ioannides

Credits: Directed by Justin Benson, Aaron Moorhead, script by Justin Benson. A Well Go Entertainment release.

Running time: 1:43