Bruce Robertson, portrayed by James McAvoy, is a deeply troubled Detective Sergeant based in Edinburgh, Scotland. Gripped by borderline personality disorder, Robertson is not only manipulative and scheming, but he also harbors a profound dislike for those around him. He neglects his duties in favor of indulgent vices, including drugs, alcohol, and sexually abusive relationships. His enjoyment comes from orchestrating cruel games at the expense of his coworkers. One of his frequent victims is his mild-mannered friend Clifford Blades, played by Eddie Marsan, whom he relentlessly pranks by calling his wife, Bunty, under false pretenses and soliciting phone sex.
Among his colleagues is Dougie Gillman, portrayed by Brian McCardie, whose extreme ideologies seemingly align more with the Third Reich than the ideal of a typical police officer. Robertson feels threatened by Peter Inglis, played by Emun Elliott, his main competitor for a coveted promotion, further complicated by the fact that Peter is openly gay. The rookie on the team, Ray Lennox, embodied by Jamie Bell, struggles with his own demons as a junkie. Robertson also must contend with Gus Bain, played by Gary Lewis, who, despite his age and ineptitude, remains a formidable rival.
Then there’s Amanda Drummond, depicted by Imogen Poots, who is determined to win favor within the department and has likely had romantic entanglements with a significant portion of her colleagues. Supervised by Bob Toal, played by John Sessions, who harbors aspirations of transitioning into screenwriting, Robertson’s actions draw unwanted attention and concern. Bob, belonging to the same masonic lodge as Robertson, is intent on having the prank caller who is tormenting Blades's wife caught, believing it could provide him with a crucial advantage in the race for promotion.
Robertson's ambition drives him to spearhead an investigation into the brutal murder of a Japanese student, which is suspected to be a hate crime perpetrated by a group of white students. Unbeknownst to Bunty, Clifford's wife, she is intrigued by Robertson when he instructs her to engage with the pervert to help flush him out. In a shocking twist, Bunty is dissatisfied with her husband, creating a dangerous connection between them.
As the investigation progresses, Robertson seeks information from a prostitute about the Neo-Nazi gang linked to the murder, mistakenly believing the case to be solved. However, circumstances shift when Bob decides to shift focus to Amanda, as public interest in the case wanes. During this tumultuous journey, Robertson struggles increasingly with hallucinations that spiral into full-blown insanity. A series of surreal conversations with Dr. Rossi, portrayed by Jim Broadbent, unveils that Robertson is grappling with the guilt of a tragic childhood incident that claimed the life of his younger brother.
Determined to press on despite his failing mental state, Robertson attempts to exclude Drummond from the investigation while clandestinely gathering DNA samples from suspects at local brothels. In a cruel maneuver, he goes on to expose Peter’s sexuality, undermining his chances for advancement in a department rife with prejudice. Additionally, the film reveals that Robertson’s estranged wife, Carole, has left him for another man, restricting his access to their daughter, Stacey. This estrangement intensifies his desperation as he becomes more erratic, even dressing as Carole off duty in an effort to feel closer to her.
In a striking turn of events, Robertson chooses to escape to England for a drug and sex-fueled getaway with Blades, while Amanda takes the lead in the ongoing murder investigation. She discovers a witness who is actually Carole and sets her sights on bringing in Gorman for questioning. In retaliation against Blades for jeopardizing his plans, Robertson frames him as the prank caller, leading to Blades's imprisonment. Tragically, Peter Inglis takes his own life following his suspension.
As Robertson roams the streets, he finds himself at the mercy of a gang led by Gorman—who orchestrated the murder—and suffers a severe beating. In a desperate act of survival, he ultimately kills Gorman by pushing him out of a window. Despite this act of bravery, Robertson’s actions lead him to not only miss out on the promotion but face demotion back to Constable with a reassignment to uniforms, while rookie Ray Lennox rises to Detective Inspector.
In a poignant moment, Blades receives a recorded apology from Robertson. Overwhelmed by his actions, Robertson prepares to end his life but is interrupted by a woman named Mary, whose husband he once attempted to save, and her son. In an unexpected twist, Robertson breaks the fourth wall, addressing the audience directly, reiterating his infamous catchphrase, "Same rules apply," while humorously laughing as the chair he stands on collapses beneath him.
