A short plot synopsis might be 'an ambitious cop jostles amongst co-workers for an upcoming promotion, but the depths of his depravity and apparent mental illness thwart him at every turn'. But Filth is both so much more and so much less.

James McAvoy is all frenetic darting eyes, anger, sneering and movement as anti-hero Bruce Robertson, an Edinburgh detective the film (and presumably the book by Irvine Welsh) is at great pains to present as a scumbag of the lowest order. He takes every kind of drug there is. He sleeps with co-workers. He sleeps with co-workers wives. He makes obscene phone calls to the pixie-ish wife of his nerdy accountant best friend (Marsan), then is put on the case to find out who the perpetrator is. When he and squadmate Ray (Jamie Bell) bust a low life with a much younger girl, Bruce offers not to arrest her in exchange for a blowjob.

After awhile, you get the idea. A plot of sorts is kicking into gear all this time, but you're not sure exactly what it is. Bruce also appears to be violently bipolar – maybe even schizophrenic – so after awhile we can hardly believe anything the movie tells us he's seeing. One of his pursuits that might be his prime concern is to get his estranged wife and son back into his life, but neither their presence nor his conviction to reach them are solid enough to mean much.

In his opening monologues about his colleagues, Bruce seems to be inhabiting a comedy, but it turns darker on a dime in amongst material designed to make you laugh, and the tone ends up a mess throughout. It also suffers narratively with most strands of the plot ending up lost in Bruce's fever dreams of drugs, sex and hallucinatory terror. Characters as well as subplots flit in and out with little consequence.

It also doesn't help that writer/director Baird doesn't have a subtle or seamless touch with such in your face material like Danny Boyle did in Trainspotting (still the yardstick by which all Irvine Welsh's other work is judged), and the whole thing comes off like a little kid who's been told he's allowed to swear for half an hour and he won't get into trouble.