Upon convincing his male co-workers at an office Christmas party to Xerox their junk and pin each picture to a wall so their female colleagues can judge whose is whose, Detective Sergeant Bruce Robertson (James McAvoy) tricks up his own photocopy to exaggerate his size. This amplification of the anatomical facts exposes Robertson’s dubiousness as Filth’s narrator, underpinned by his occasional breaking of the fourth wall, whose effect is to cast doubt on the feverish twists taken by Jon S. Baird’s film. Despite being lewdly mischievous and a misogynist, Robertson views himself as the only competent bloke in the whole Edinburgh precinct and the hero of his own story. Yet as the film throbs toward its third act, where Robertson’s entire existence spirals into hallucinatory madness, it becomes clear that the world he mentally envisions differs vastly from the one in which he actually lives.
Robertson is tasked with investigating the murder of a Japanese student, but it immediately becomes clear that he’s more interested in pursuing a sordid lifestyle of booze, drugs, sex, and self-pleasure. Thirsting for advancement within the department, he feels no remorse for fucking over his colleagues and literally fucking the ones they love, though his motivation seems less tied to promotion than manic amusement. He repeatedly phones the wife (Shirley Henderson) of his so-called best friend (Eddie Marsan), an over-earnest department accountant and deliberate altruistic counterpoint to the movie’s overall narcissism, talking dirty but disguising his voice. When she complains to the police, Robertson’s chief orders him to uncover the identity of the pervert calling her, meaning he’s essentially made to search for himself. This signals that Robertson will be undone not by outside forces, but by demons lurking within.
Those demons are assigned common causes: a wife who left him, a young son he never sees, and a past tragedy involving a younger brother. It’s an unconvincing psychology for a character when the film argues his real issue is psychosis. And at convenient moments throughout the film, Robertson encounters a young mother and son—stand-ins for the family he pushed away and symbols of his possible salvation. Ultimately, however, Filth is too nihilistic to believe its protagonist can be saved, declaring him a lost soul and satisfied to let him suffer. And in spite of the character’s repulsiveness, McAvoy, gleefully losing himself in the metaphorical filth of the title, makes the eternal torment bizarrely absorbing.