Filth is the story of detective sergeant, Bruce Robertson (McAvoy) of the Lothians and Borders police force. He is a misanthropic, misogynistic, bigoted, corrupted, drug taking, alcoholic, scheming monster. In fact his terrible traits could go on for another paragraph.

The film opens with a racially motivated murder of a Japanese foreign student. We are introduced to Bruce Robertson who has been assigned to investigate the murder. The story is told in the first person perspective and occasionally through the perspective of the “tapeworm” growing inside his intestines asking existential questions.

He is involved in a bitter contest with work colleagues for a vacant detective inspector job. In the brilliantly executed scene, we are introduced to his competitors with their odds for promotion flashing up on the screen. Ray Lennox (Jamie Bell) is the new kid on the block and rank outsider for promotion; but as we’ve learnt never underestimate the underdog.

Soon we travel through the sordid life of Bruce Robertson, whether he is scheming against his colleagues, making sexually harassing phone calls to his best friend’s wife, getting high on cocaine with Ray and having sex with an underage drug dealer’s girlfriend.

The film isn’t easy viewing and the title aptly describes the subject matter. When the book was made into a play in 2000, on the opening night, two of Lothian and Borders’ finest had to be called in to eject a heckler. No doubt there will be cinemagoers disgusted with the film.

What makes this film unmissable viewing is McAvoy’s devastatingly poignant performance. He gives a brutally honest performance. He’s not portraying some psychotic character that exists only on screen but a flesh and bone human being suffering the traumas of a failed marriage and bipolar. Instead of feeling repulsed by the character of Bruce Robertson, you feel genuinely empathy towards him by the end of the film.

Director Jon S. Baird has adapted and directed the source material with respect and true to the spirit of Irvine Welsh’s book. In fact, he is great friends with Welsh, so you would not expect anything less of him.

James McAvoy’s outstanding performance aside, he’s ably supported by Jamie Bell as a deceptively naïve looking sidekick. Imogen Poots plays Amanda Drummond, another one of Bruce’s rivals for promotion, and brings out the worst side of his character’s sexism and ultimately his vulnerability. Eddie Marsan is suitably nerdy playing Bruce’s long suffering accountant best friend, Bladesy.

The twist at the end of the film might be predictable but gives a satisfying explanation to how Bruce Robertson became the character he is. But don’t take it from us; the author Irvine Welsh has proclaimed James McAvoy’s performance as better than Robert DeNiro’s in Taxi Driver. With no less than four films coming out in 2013, it definitely looks like James McAvoy’s year.

Released in UK cinemas on Friday 4 October 2013.