Filth doesn’t just lift the rock to peer at Scotland’s sordid underbelly, it kicks the rock over, drops its trousers and pisses all over it.

Typical: you wait ages for a film about Scotland and then two come along at once. But where “Sunshine On Leith” was the happy, charming, airy Dr Jekyll, “Filth” is a dark, twisted, wanton Mr Hyde.

“Filth” doesn’t just lift the rock to peer at Scotland’s sordid underbelly, it kicks the rock over, drops its trousers and pisses all over it. This is a film which is not afraid to shock and has absolutely no hesitancy in showing the degradation and abject desperation of its motley cast of characters.

This being Irvine Welsh, the whole heady cocktail of sex, drugs, violence and swearing is served up with lashings of the blackest humour you’ll ever see on screen. But the trailer barely scratches the surface of this dark, complex and riveting movie. If, based on the trailer, you’re just expecting a drug-fuelled pitch-black comedic romp then you are in for a surprise.

Darker and more vicious than its spiritual predecessor “Trainspotting”, “Filth” tells the story of Bruce Robertson, a corrupt Edinburgh police detective scheming and tripping his way through Christmas as he tries to secure a promotion which he believes will help him win back his wife and daughter while his grip on his sanity slowly disintegrates. “It’s A Wonderful Life” this is not.

There are wonderful supporting performances from Jim Broadbent as an increasingly surreal Australian doctor, Eddie Marsan as timid accountant Bladesey and Jamie Bell as coked-out rookie cop Ray Lennox, all of whom become props to keep Bruce staggering forward in his progressively deranged and misguided quest for promotion and, ultimately, redemption. Amongst great turns by Emun Elliott, Ron Donachie and Gary Lewis as the other members of Bruce’s department, John Sessions also impresses as the clueless, pompous blowhard Chief Inspector Bob Toal, a greasily bloated performance that more than tips the hat to Alex Salmond.

But as good as the supporting cast are – and boy are they good – “Filth” is an absolute tour de force from James McAvoy. He is simply astonishing in the role of Bruce Robertson, effortlessly moving from hateful to heart-breaking and back again with consummate ease and skill. Robertson is a disgusting, manipulative, sociopathic deviant yet, thanks to McAvoy, he’s never anything less than a sympathetic presence, even when at his lowest.

Screenwriter and director Jon S Baird infuses the film with a vibrancy which lends an almost hyper-realistic sheen to the grey and grimy locations and gives the whole affair a vaguely hallucinatory air, pushing us forward, along with Bruce, down a path we increasingly fear to tread. If you’ve ever wondered what a Moaning Myrtle sex line would be like, “Filth” is the film that will finally give you the answer.

Bleak, profane and savagely funny, with James McAvoy revelling in the gob-smacking debauchery, “Filth” is one of the must-see films of 2013. But it’s not for the faint of heart. You’ll definitely laugh, you’ll probably gasp, you might even jump and there’s a chance you could shed a tear but I guarantee you this: you will be entertained.