Investigation of a Citizen Above Suspicion, made by Italian director Elio Petri in 1970, is a unique film. Superficially, it’s an unusual, occasionally decadent, sex-crime story about the murder of a top policeman’s lover. But that’s only a McGuffin for Petri’s deeper social, political and psychological concerns.

The film is a scathing, hard-hitting political tract about state corruption, police brutality and fascism, as well as a penetrating psychological analysis of a mind gone awry. It also takes a philosophical look at notions of justice, and the difficulties of achieving a just society in the face of human fallibility.

Despite its intellectual aims, Investigation is never ponderous or polemical, being leavened with a deathly black humour and some bizarre, semi-surreal scenes. A best foreign language film Oscar winner, it starts with the chief inspector of the homicide squad (who is not named, but referred to as “Il Dottore”) killing his debauched mistress and littering her apartment with clues that point to his guilt.

Leaving the homicide department to take command of the Gestapo-like political police, the psychologically disturbed Il Dottore attempts, in an equivocal fashion, to get arrested for the murder. But a mixture of incompetence, fear, disbelief and unwilling­ness to see the obvious, means that his ex-colleagues in the homicide squad fail to bring him in for the crime. Will Il Dottore have to confront them directly with his guilt – and even then, will they accept it?

A brilliant performance by Gian Maria Volontè as Il Dottore is the linchpin of the film. The script demands that Volontè pitches his character somewhere between realism and surrealism – he has to appear comfort­able in the real world of the police depart­ment, yet deranged in his inner life. That’s a tough call for any actor, but Volontè skilfully unites the two sides of this original character with a manic gusto. Interestingly, although Volontè plays a fascist cop here, he was in reality a well-known militant leftist.

Petri’s aim was to make left-wing films within the studio system, and he went on to direct three more studio movies with highly political themes. With hindsight, he seems to have fulfilled his goal with Investigation, and that’s no mean triumph, considering that the repressive society he depicts really did exist in Italy in the 70s. At the time, his decision to work inside the capitalist studio system brought much criticism from the left.

Seen out of context 46 years later, the film can simply be enjoyed as a masterpiece of cinema, although its themes still apply to many states today.

Investigation of a Citizen Above Suspicion will be screened on September 16 at the Hong Kong Film Archive, in Sai Wan Ho, and on October 8 at Festival Grand Cinema, in Kowloon Tong, as part of the Cine Fan programme.