This is the way it used to work, the image we used to be famous for.

An accused man is locked up on suspicion of his involvement with the 9/11 hijackers. Lawyers hear about him, locked up at the prison on Guantanamo Bay, not charged, and take up his case.

Because as a country that the world expects to operate under the rule of law, you don’t keep somebody locked up for years, interrogated above and beyond the pale without someone getting outraged, without somebody crying out for justice.

“The Mauritanian” is about an infamous case from the post-9/11 Bush years. Mohamedou Slahi was approached at a family party by his country’s police, told “the Americans” wanted to question him…and disappeared. For years.

Kevin Macdonald (“The Last King of Scotland”) has directed a factual, well-acted and somewhat laborious look at his story, a detailed movie that runs out of time rather abruptly at the two hour mark and shoves a summary finale in that skips over years of Obama administration involvement.

So it’s fascinating but flawed, more truncated than thorough.

Tahar Rahim (“A Prophet,” “The Kindness of Strangers”) plays Slahi, a guy we see just enough of to wonder about his innocence. He lives in a country where the police are feared, but does what he can to protect himself when they come calling.

He’s allowed to change clothes, which allows him to erase his phone’s contact list and call history. And he gets to drive his own car to the station.

It’s November of 2001 and we’re not given a lot of information. “Would they let me drive my car if I wasn’t coming back?” he reassures his mother (in Arabic).

But clearing one’s phone seems suspicious.

Jodie Foster plays Nancy Hollander, an Albuquerque attorney who’s “been fighting the government since Vietnam.” She’s a bit of a ball-buster — older, her name’s on the firm, and she’s not shy about taking on an ACLU case. It’s been over three years and this guy’s family wonders what happened to him. He might be among the 7-800 detainees at “Gitmo.”

Calling around only gets her a “He’s not not here” answer. That’s great detail, showing just how hidden from the world these prisoners were and how hard it was to even learn who was there.

With a young associate (Shailene Woodley), Hollander gets on a plane to Cuba, struggles to win the trust of this man who doesn’t trust anything associated with America, and becomes his lawyer. She will fight for a writ of habeas corpus — a demand to know what the government has on him and if it has the right to keep holding him without charge.

Meanwhile, the Bush administration has decided to pursue legal precedents that allow them to put Slahi on trial for his Colonel assigned the job of prosecuting Slahi, whom they accuse of helping recruit the 9/11 hijackers.

“Rough justice, that’s what this administration wants,” he’s told. Get it for them.

As each side digs into the case, facing “classified” and “redacted” material at every turn, Slahi writes a long account, over several long letters to his lawyer, telling his story. Flashbacks from those letters recreate his life — a bright kid given a scholarship in Germany, outrage over the Soviet invasion of Afghanistan, traveling there.

Macdonald, working from a three-credited-writers screenplay (with one writer taking the name of “Treasure of Sierra Madre” scribe B. Traven), may struggle to wrestle this into a compact narrative. The film has a geographical and sociolinguist disconnect. It never really feels like we’re watching an American story playing out in the US. That’s jarring, but by design.

The performances, the scenes recreated, the dialogue and the detail immerse us in this case even if we get lost here and there.

“Even in Mauretania we have watched ‘Law & Order’ and ‘Ally McBeal,'” Slahi says at one point. But the legalese here, the blizzard of legal and military acronyms, scenes of wholly-redacted paperwork in a top secret government archive, is enough to test even lawyers watching this.

Scenes of what Slahi went through — endless interrogations, torture — aren’t as rough as many films depict them. But the process and procedure as seen here is fascinating. Two questioners and an interpreter all play “good cop,” at first. And then one by one they turn hard.

Foster gives Hollander a brusque, clipped demeanor, impatient with anyone who blows her off or minions who can’t keep their feelings out of the case. Call her a “terrorist lawyer” and this woman has the perfect comeback.

Woodley, collecting credits with every Oscar winning actress under the sun, holds her own. Rahim has some nice moments, but his character doesn’t have enough scenes to properly engender sympathy. And Cumberbatch does a good job with Couch’s soft Southern accent even if there’s not enough screen time to develop the man’s faith-backed decision making.

The players make the whole enterprise watchable and worth taking a look at, just to remember that the erosion of America’s “rule of law” reputation didn’t start with the outgoing administration in Washington. It’s been withering under three presidents.

But the screenplay needed more work and the film in the can a lot more editing to make “The Mauritanian” worthy of the talent on the set.

MPA Rating: R, violence, profanity

Cast: Tahar Rahim, Jodie Foster, Benedict Cumberbatch and Shailene Woodley

Credits: Kevin Macdonald, script by Michael Bronner (M.B. Traven), Rory Haines and Sohrab Noshirvani. An STX release.

Running time: 2:08