Any movie that reminds us of the ongoing civil rights scandal at the US’s extrajudicial detention camp at Guantánamo Bay should be a good thing: it’s still open for business right now, with 40 prisoners inside. The same goes for any reminder of the 9/11 terrorist outrage and the backlash of furious revenge it was designed to provoke, implanting a virus of rage and fear that threatens to live on in the American bloodstream like malaria.

But I was disappointed by this well-meaning movie, based on the true story of Mohamedou Ould Slahi from Mauritania in north-west Africa. A former muhajideen anti-communist fighter in Afghanistan in the 1990s, he was picked up and handed over to the US authorities after 9/11 (with the Mauritanian government’s permission) and kept at Guantánamo Bay without charge or trial for a staggering 14 years, from 2002 to 2016; he was released when the state finally accepted his confessions were valueless, having been obtained through torture.

The film is adapted by screenwriters MB Traven, Rory Haines and Sohrab Noshirvani from Slahi’s book, Guantánamo Diary, published in 2015 while he was still inside: the scribbled pages regularly handed to his lawyer Nancy Hollander. Franco-Algerian star Tahar Rahim plays Slahi; Jodie Foster plays Hollander and Shailene Woodley is her associate, Teri Duncan. Benedict Cumberbatch plays the crewcut military prosecutor Lt Col Stuart Couch, who was pretty gung-ho about getting the death penalty for his man until he realised that it meant relying on torture and disregarding the constitution and the rule of law.

So far, so admirable. But with this movie, we are plunged right back into the exasperating 9/11 fence-sitting handwringer genre that was fashionable in the 00s: conscience-stricken films that invited us to sympathise with their liberal agony, such as Robert Redford’s Lions for Lambs (2007), Gavin Hood’s Rendition (2007) and Stephen Gaghan’s Syriana (2005).

The Mauritanian is a movie that appears to be comprised entirely of good guys: Slahi himself is a good guy, of course, and so naturally are Hollander and Duncan, doggedly ploughing through the boxes of legal documents that the authorities allow them to see, and persistently asking for more. But the chief prosecutor Couch is a good guy as well, troubled with his finally overwhelming qualms of conscience as a true patriot. (Hollander and Couch are shown having a reasonably cordial beer together at the Guantánamo visitors’ cafe.) Finally, Slahi gets his day in court in which, with stirring music on the soundtrack, he praises American TV shows such as Ally McBeal and American justice itself.

So with all these potent good guys effectively rooting for the prisoner, why did he stay banged up for so long? There are no major players on the bad guy team here: authoritarian meanies are permitted on screen on condition that they are dramatically dominated by a liberal convert: Cumberbatch. There is nothing and no one in this film with the dramatic status of, say, Jack Nicholson’s ferociously unrepentant Colonel Nathan Jessup in A Few Good Men, scripted by Aaron Sorkin, and there is no “you can’t handle the truth” moment. There is just official silence from the authorities and the drama itself; a sombre announcement flashes up on screen that Slahi stayed in Guantánamo for six years after the prosecution collapsed in 2010 – by order of the Obama government. As for Slahi himself, he doesn’t seem bitter about the US nor Mauritanian authorities by the end of the picture; he doesn’t wish to take action against them, yet neither does he explicitly forgive them.

It’s opaque and frustrating. Rahim gives a perfectly decent performance and everyone else does an honest job. Slahi himself is throroughly entitled to his own happy ending, cheerfully listening to Bob Dylan over the closing credits. But this movie is content with congratulating itself for being on the right side of history, with little attention paid to questions unanswered and history unresolved.

• The Mauritanian is released on 19 February in the US, and in the UK on 2 April on digital platforms.