The Mauritanian might not be quite as good a film if it were not based on real events. The tragic and horrifying story of justice denied to a political prisoner, complete with corrupt officials, scheming wardens, a complicit guard uncovering the truth and finding his conscience, a crusading attorney battling the odds, and the ultimate – if belated – victory would be considered an unlikely, hard to swallow melodrama pandering to the public’s fears and sympathies, if it were fiction. Sadly, it is not fiction. The script, by the writing of team Michael Bronner, Rory Haines, and Sohrab Noshirvan, was drawn from the bestselling memoir Guantanamo Diary by former prisoner Mohamedou Slahi.

Originally from Mauritania, Slahi received a scholarship to study in Germany, where he became an electrical engineer. The film barely touches on the ensuing few years, but Slahi attracted the suspicion of American authorities, partly due to a relative who was involved with al Qaeda, and a terrorist who attended the same mosque as Slahi. He was repeatedly questioned over the years, and in 2002, after American distrust was enhanced by the 2001 terrorist attacks, Slahi was imprisoned by the US military at the detention camp at Guantanamo Bay, Cuba.

The film opens in 2002, when Slahi, played by French actor Tahar Rahim, is in Mauritania to attend a wedding. The local authorities ask him to come with them, to speak to American officials who want to question him. Accustomed to this kind of suspicion, Slahi goes along, and from the perspective of the friends and relations, he was left behind, disappearing for several years. He is shortly brought to the notorious facility at Guantanamo Bay, and that is where the central storyline begins.

Much of the film is essentially a legal drama, although unusually heavy on the drama and on the weighty human rights issues connected to the case. Attorney Nancy Hollander (played by Jodie Foster), aided by junior lawyer Teri Duncan (Shailene Woodley), volunteer to represent Slahi, a man who has been incarcerated for many years without outside contact, without the benefits customarily granted to prisoners by either American law or the Geneva Conventions, and without even having been charged with a crime. Hollander finds herself working against tremendous resistance from her client’s captors, who systematically attempt to stall the process, severely limiting her contact with Slahi, withholding any evidence in their possession, and using highly unethical means to sabotage his defence.

The well-organised plot presents us with several parallel, neatly intertwined storylines, each containing significant information which is gradually revealed. It follows the complicated and frustrating legal case which, over many years, worked to give Slahi the right to an actual court trial and to gain his exoneration and release from prison. In the process, it reveals the behind-the-scenes efforts – often legally and ethically questionable efforts – by the US military to suppress any information about Slahi’s case, and to ensure he remains in prison, indirectly informing us about the questionable history and status of the Guantanamo Bay installation. Finally, as Hollander gains her client’s trust, his experiences since being taken into custody, including his ‘enhanced interrogation,’ are disclosed.

Character development is a big part of the film. Mohamedou Slahi begins as an unknown; we know his character and background mainly based on the unshakable conviction of his captors that he is a terrorist, a conspirator with those who attacked US sites in 2001. Even his lawyer does not, at first, believe in his innocence, only in his right to a trial. As the legal aspects of the story unfold, we gradually learn more about Hollander and her associates, about the prison staff, even about the other detainees, who can be heard as they communicate with Slahi but remain unseen. Most of all, we learn about Slahi himself, his background, the real details of the supposed case against him, and who he is as a person.

Only toward the end of the film are we shown the specifics of Slahi’s long series of interrogations while he was imprisoned, often referenced by never fully described until Hollander urges her client to provide the data and put it in writing. The film presents these events as a series of flashbacks, filmed in a shadowy, dreamlike form and revealing only brief flashes of the action, in a way that mutes the horror of his systematic torture, permitted under recently established government guidelines, without completely hiding the gruesome details. It is a highly effective climax to the story thus far, and puts a great deal about Slahi and his manner in context.

An additional subplot helps to clarify the situation, as well as add human interest. Benedict Cumberbatch plays military lawyer Stuart Couch, who is assigned to prosecute Slahi and urged to ensure he is not released under any circumstances, even if it involves “rough justice’. Couch takes on, in a sense, the part of the presumed audience, largely uninformed but suspicious of Slahi and slowly becoming aware of the truth. He is enthusiastic about his assignment at first, convinced of Slahi’s guilt and quietly hostile toward the terrorists who attacked his country and killed one of his friends in the 9/11 attacks. As the story unfolds, he learns more about Slahi’s arrest and studies the evidence against him, becoming more and more uneasy with what he discovers. The scenes of Slahi’s recalled experiences of humiliation and torture coincide with a climactic scene in which Couch reads the account, and becomes disillusioned, then horrified by the level of subterfuge and morally questionable actions involved in the Guantanamo Bay prison. His personal and ethical dilemma is gradually resolved as the case matures.

What might, in a fictional legal drama, be presented as a victorious happy ending instead becomes a carefully managed bit of satirical commentary in the final act, with each apparent gain contrasted by a loss, as the US government intercepts or forestalls every achievement by Slahi and his lawyers. The film concludes with the usual onscreen texts describing the outcome of each character, but providing more information than usual, and concluding with a series of real-life photographs and videos that bring attention to the real people involved in the situation and the real suffering endured by them.

An excellent ensemble cast, and the undeniably fascinating facts of the case, keep the film engrossing throughout. It is helped along by the script, which allows all the pertinent information to be revealed slowly, maintaining suspense throughout and keeping a sense of mystery despite its occasionally documentary-like approach. Still, more important is the management by director Kevin Macdonald, whose skill with suspense is well known from previous films like The Last King of Scotland and State of Play, and who makes wonderful use of set design to set a tone and help tell the story. Most significant of all is the performance of Tahar Rahim as Slahi, a demanding role which requires him to portray, at different times, an ordinary, free man; a subject of incredible abuse driven to a subhuman level of humiliation, confusion, and pain; and a cautiously hopeful prisoner, unsure of whom he can trust, working his way through a hostile bureaucratic labyrinth. His remarkable performance is what puts this film over the top, and captures the real human being behind the story.