“American Trial: The Eric Garner Story” is a righteous cause supported by a very bad idea, but a bad idea (a docu-drama) executed about as well as could be expected.

The “cause” is getting us to take another look at a 2014 case, an unarmed, overweight black man tackled, arrested and put in a choke hold by cops intent on arresting someone for selling loose cigarettes on a Staten Island street corner.

It was, as lawyer, legal scholar and Fox News talking head Alan Dershowitz says, an “alleged offense so trivial” as to beggar belief, with consequences — Garner died from injuries suffered during that illegal (NYPD banned “choke hold”) — that could not have been more dire.

A Staten Island grand jury elected not to recommend prosecution of NYPD officer Daniel Pantaleo, the man who applied the chokehold, and has never released the transcripts of the evidence seen and heard and their deliberations.

If you’ve seen the video of the arrest, four officers surrounding the gigantic, protesting Garner, nobody once saying “I’m gonna have to write you a ticket,” taking the man to the ground to the shouts of “I didn’t do NOTHING, no no no NO NO I can’t breathe, I can’t BREATHE,” a phrase he gasps out eleven times, you can’t ever forget it.

“Excessive force,” you think, at the very least. “I just saw a man murdered” might be a reasonable conclusion, too.

The case, one of a distressingly large number of such cases (continuing to this day) of armed cops killing unarmed civilians, became a national flashpoint. “I can’t breathe” T-shirts made it all the way to the NBA.

Anything that brings attention back to this seeming injustice is justified, right?

But filmmaker Roee Messinger elected to do it via a movie, a “What if there’d been a trial?” take on the case. This is the “very bad idea.” Messinger decided to make a docu-drama, not a documentary.

Using the known evidence, attorneys hired to prosecute and defend the case, a lawyer hired to play a judge, real witnesses — Garner’s widow, Esaw, a friend, retired NYPD officers and medical examiners speaking on behalf of the defense and the prosecution, and an actor hired to be Pantaleo (Anthony Altieri) — inventing pre-“trial” interviews and a rented courtroom, Messinger runs a mock trial.

And while that can be a most instructive exercise for training law students, it makes for generally dull and a disastrously deceptive “what might have been” made in pursuit of “justice.”

Blending in actual news coverage of the trial, footage of LeBron James and teammates in Garner “I CAN’T BREATHE” jerseys, then-President Obama speaking out, TV talking heads talking up the subject, just amplifies the crime against objective truth.

If you don’t remember who said what (Republican Rep. Peter King is here), how do we know what was really said, and what’s imagined for the cameras? An opening “nothing was scripted” here explanation doesn’t cover it.

What can we believe?

Compounding the film’s problems is the fact that most of it consists of this mock trial. Legal junkies obsess over big cases — Casey Anthony and George Zimmerman here in my (Florida) neck of the woods, for instance. But like most court cases, this is visually and dramatically dry and dull, with factually defensible testimony and spurious “objections,” cursory opening remarks and perfectly logical and believable closing statements.

You’re here to try decide the guilt or innocence of “Danny,” the actress/lawyer playing the defense attorney intones, “NOT here to decide ‘Black Lives Matter.’ They do.” She quotes Martin Luther King Jr. and Gandhi, too.

I could totally see and hear that happening. But it didn’t. It hasn’t. And “American Trial” seems a blundering attempt to change that fact.

Movies have turned a spotlight on injustices before, and some — the various documentaries on the West Memphis Three, Errol Morris’s “The Thin Blue Line” — have led to justice being done. Such films often include recreations of the crimes being discussed.

But here’s a tip somebody should have passed on to filmmaker Messinger before he rounded up the GoFundMe cash to make “The Eric Garner Story,” which really isn’t” The Eric Garner Story,” by the way.

You want to move the legal system, you stick with the facts and the compelling tragedy and grave injustice those facts support.

Nobody EVER got a new trial, a re-hearing in the bright light of public opinion, via a docu-drama.

MPAA Rating: unrated, disturbingly violent video

Cast: Esaw Garner, Alan Dershowitz, Anthony Altieri

Credits: Directed by Roee Messinger. A Passion River release.

Running time: 1:40