A Prophet is good. Any review will tell you that. It snatched the Cannes Grand Prix. It is acted immaculately. Its barbaric moments are filmed with staggering beauty. Its subtle virtue, however, is its accuracy in portraying human ambiguities. If you walk away chilled, it’s not for an emotionless passage, but for the impossibility of favouring one character’s triumph over another’s downfall.

This is the anti-Shawshank, a grim prison epic spanning the six year sentence of a young French Arab. It’s not Malik’s sentence which interests Jacques Audiard: it’s his passage from inarticulate pawn in the prison yard to calculating king of internal favours and external crime. The journey is catalysed by Corsican crimelord César, who offers Malik protection in exchange for the murder of fellow inmate Rayeb. In the tensest twenty minutes of film you’re likely to witness this year, Malik must recast himself as murderer in mind and body. The retaining of a blade between the gums is not an easy practice to swallow, and the eventual encounter is as horrifically vivid as Viggo Mortensen’s bathhouse confrontation in Eastern Promises.

Tahar Rahim conceals his character behind inscrutable resignation. Ever-watching, Malik is as unknowable as the cinemagoer besides you, a curious whelp whose subservience bursts sporadically towards rebellion. Such minimalist disclosure feels suitably cold. Characters and events are introduced by a paused screen and bold titles, fleeting chapters in the Dickensian bombardment of prison life. The stylish Tarantino slices cut a neat parallel with the film’s tenderness, for even César is pitiable, stagnating, old and alone in his privileged cell. Just when A Prophet is unbearably bleak, however, it indulges a strange element of the fantastical. Rayeb is a constant presence in Malik’s cell. The ghost is sometimes stern, sometimes cackling; his throat remains slit in each encounter. Audiard could so easily have created a symbol of guilt, but Rayeb is a confidant, and the catalyst for Malik’s bemusing moments of psychic insight which offer the film its title.

Mysticism, crime, tragedy: sometimes A Prophet yearns for its epic status with too dogged a desperation. The intricacies of double deals and broken loyalties aren’t easy to keep in check; better to let them play out in sleek exchange after sleek exchange. Difficult to watch for its bleakness and its complexity, A Prophet is made more unbearable – and impressive – in its insight of men, none of whom secure sympathy; each of whose souls are impossible to define.