Featuring a standout, awards-worthy performance by Tahir Rahim, Kevin Macdonald’sThe Mauritanian,” adapted from Mohamedou Ould Salahi’s memoir “Guantanamo Diary,” about his prolonged imprisonment at the titular base, is a staggering work of docudrama, that highlights the horrors that the American government inflicted on terrorist suspects post-9/11. 

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Rahim plays Salahi, a Mauritanian who lives in Germany and, post-9/11, is arrested and renditioned to Guantanamo Bay for three years on vague accusations of orchestrating the attacks. After the Supreme Court decides that prisoners cannot be held without legal representation, Nancy Hollander (Jodie Foster – shading in degrees of rightful outrage), a New Mexico based human rights lawyer, and her assistant, Teri Duncan (Shailene Woodley –  a welcome audience surrogate), take up Salahi’s case pro-bono. Unsure of his innocence, they plan to defend Salahi on civil rights violations. On the other side, Lt. Stuart Couch (Benedict Cumberbatch – stoic if a bit shaky in his accent) prepares his case against Salahi, uncovering the “special projects” that the prisoner was subject to. 

What begins as an engrossing, if perhaps typical, investigatory film quickly splinters as Macdonald, and screenwriters M.B. Traven, Rory Haines, and Sohrab Noshirvani, cross-cut between past and present, using Salahi’s journals, which he sends to Nancy and Teri, and would become the basis of his book, as a framing device. Filmed in a grainy 4:3 ratio that boxes Salahi and the viewer into the cells and interrogation rooms that he moves between, these interludes are deeply troubling. As days, weeks, and eventually, years start to blend together for Salahi, it becomes frankly obvious that anyone would admit to anything to avoid the horrors that he had to endure. 

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Further, Macdonald juxtaposes Hollander and Couch, who are not at odds with each other as first hinted at. Hollander, a crusader who allows Foster to anchor the film, if not fully explore her acting range, is exactly the moralizing do-gooder that she initially seems, but at a cost to her personal life. Couch, a deeply religious and moral man, may lead the prosecution, but he ascribes to a higher truth, something that would, honestly, be unbelievable if it weren’t rooted in facts. 

With all these competing storylines in place, Macdonald deftly moves between them, highlighting the grueling work that both Hollander and Couch undertake, centralizing the drab office spaces that they move between, as Salahi languishes in a different kind of space, unsure if will ever see freedom again. Yet, unlike similar polemical courtroom dramas (Gavin Hood’s similarly themed “Official Secrets” perhaps being the most recent), “The Mauritanian” doesn’t climax with legal grandstanding. In fact, Macdonald wisely eschews most of these scenes, instead choosing to culminate the film with Salahi’s torture, and Couch and Hollander’s realization of what exactly Donald Rumsfeld meant by “enhanced interrogation techniques”

When Macdonald finally showcases the horrors that Salahi had to endure, as Hollander reads Salahi’s journal and Couch looks over the CIA files, the film veers into Lynch-ian horror show, literalizing Salahi’s splintering psyche. As guards, wearing anthropomorphized masks, inflict horrors on Salahi’s body, degrading him in any number of ways, and threatening his family, he and the film regress into his past, showcasing the mutability of memory and the futility of physical persecution. Unlike the rest of the film, which often plays like a slick procedural, it’s harrowing and traumatizing, far from anything that comes before or after it. 

In these scenes, and really the whole film, Rahim, who previously worked with Macdonald on “The Eagle,” and turned in another stunning performance in “The Looming Tower,” embodies Salahi’s contradictory moods. Despite the terror he endured, Salahi comes out as a humane person, unwilling to let his humanity be taken away from him. It’s truly a powerhouse performance that might not get the awards attention it deserves but stands as another reason to watch anything Rahim is in. 

Macdonald, who has oscillated between documentary and feature throughout his career, has often directed polished, if somewhat forgettable fictional films (“The Last King of Scotland” and “State of Play” being prime examples). “The Mauritanian” pulls from his work on political documentaries, including his Oscar-winning “One Day in September,” making it the best feature he’s directed, so far. Working alongside cinematographer Alwin H. Küchler (often a collaborator of Danny Boyle), “The Mauritanian” plays with aspect ratios and film stock in a way that feels experimental but never distractingly so. 

It’s no spoiler to say that Salahi is eventually released, though the circuitous route he takes to get there is another condemnation of the fractured American justice system and highlights the inability of narrative film to truly encompass a years-long journey. In the end, “The Mauritanian” is an efficient procedural that condemns the Bush-era treatment of detainees more effectively than any other recent narrative film. It’s an affecting, but nevertheless tragic, watch. [A-]

“The Mauritanian” arrives in theaters on February 19.