James McAvoy is a good actor. He’s usually playing the good guy, from the excellent biopic The Last King of Scotland to the only-slightly-less-realistic (ahem) X-Men prequels, but in Filth he gets his chance to ditch the morals (who needs ’em?) and become the straight-up despicable Scotland detective Bruce Robertson. Bruce, drug-addled and sex-addicted liar extraordinaire, will do pretty much anything to get a promotion at his job. And McAvoy relishes in the ceilingless grandeur of such a character – but without him Filth isn’t much for originality.

There are two kinds of twist endings in film: those that are truly original and those that are a rehash of Fight Club. In all seriousness, the protagonist-repressing-important-plot-points “twist” is officially tired as hell, and in Filth the rote deployment in Act Three is borderline maddening. The relative believability of the effectiveness of Bruce’s manipulations throughout Filth is what makes them so revolting. Bruce starts hallucinating more and more, and this also lends an interesting angle to his uncivilized crusade – is he as smart as he thinks? Is he as smart as we think? Or has everyone been on to him the entire time? But, oh, wait, nope – the hallucinations actually mean something more, something deep, man. Bruce is deep because he has a Tyler Durden-esque trauma trigger. Get it?

So: ditch the stupid ending and the would-be aha! moments, and Filth is pretty great in a disgusting sort of way. It’s essentially Wolf of Wall Street for McAvoy, a time to go absolutely bonkers and meanwhile find some sort of way to make Bruce somewhat likable, even if it’s just a few percent out of the larger character pie. He does just that, and perhaps it’s the fact that Bruce has some sort of a mission (to get the promotion) that it’s not hard to go along with him on the disgusting rollercoaster of drinking and smoking and sexing and snorting and sexing some more. He has a goal and a lack of morality means he can achieve the goal in any possible way, and it’s fun to watch the dominoes fall uselessly before him.

But the mission seemingly dries up as the all-hallowed twist approaches, and eventually the promotion is given away and Bruce is just doing insane shit because he can. The hope may have been that the twist would carry the momentum through to the end after the aforementioned mission is moot. It doesn’t. McAvoy is likable as Bruce because he goes all the way for the character and he’s good at what he does; the insistence on writing something deeper into his character that manifests itself as a freakin’ hallucination very nearly undoes all of his hard work, and very nearly forces him to stand in line with every other protagonist with the exact same affliction rather than standing out from the crowd.

Filth is worth it for the lead performance alone, and really the final letdown is only so disappointing because McAvoy is so spectacular. Guys like Jamie Bell are in there, too, but are pushed aside. It’s The McAvoy Show. More films would probably benefit from being The McAvoy Show.