Sandy Duffy, a famous author, wrote his well-known and only novel, A Patch of Fog, nearly 25 years ago. He now works as a university lecturer and hosts a talk show, alongside his girlfriend, Lucy Walsh. Despite the enduring success of his lone publication, Sandy has developed a shoplifting addiction and is caught by a security guard, Robert Green.
Robert detains Sandy, shows him CCTV footage, and threatens to call the police. Sandy pleads, and Robert agrees not to report him if they go for a drink together. They plan to meet that weekend, at which point Robert will pass over the disk containing the video.
After a university lecture, Sandy tells a colleague about Robert, calling him a “sad little man.” > "sad little man." The moment is sharp and revealing, signaling how unsettled the relationship has become. Robert then walks into the classroom, apparently having eavesdropped on the conversation, and leads Sandy to the railway tracks, offering a coin flattened by a passing train and keeping the other as a grim symbol of their new friendship.
Sandy grows increasingly concerned with Robert's behavior and obsession. They agree to meet at the pub, argue, and Sandy threatens to tell the police; Robert counters with his own threats. They walk to the police station together, where Robert reveals that he has copies of Sandy's other shoplifting incidents. Sandy apologizes for threatening him, and they leave the station.
Sandy invites Robert to his house and gives him a tour. He explains that he doesn’t have a copy of his famous novel, but the original manuscript is somewhere in the house. Sandy gets Robert high with a marijuana cigarette and pretends to connect with him. He asks Robert to hand over the recordings, but Robert refuses.
Desperate, Sandy resorts to more extreme measures. He invites Robert to a university lecture, but the lecturer informs him that Sandy called in sick. Meanwhile, Sandy drives to Robert's apartment, breaks in through a back window, finds the disks, destroys the apartment, and throws the disks into the River Lagan.
Feeling relieved, Sandy resumes his life teaching and hosting the show. Then a message from Robert arrives, containing security footage from Robert's flat that clearly shows Sandy's crime. Sandy drives to Robert's flat, begging for forgiveness. Robert shows up at the train tracks, distraught over the deception because he genuinely thought Sandy was his friend. He steps in front of an oncoming train, but Sandy tackles him and saves him.
On Robert's next shift at the store, he recognizes Lucy from the talk show and introduces himself to Lucy and her daughter, Phoebe. Phoebe drops something, and Robert discovers the deformed coin he had given Sandy. Enraged, he later smashes their car windows.
The next day, Sandy arrives to find the living room emptied of familiar furniture and reordered with a new set, while the couch remains. He tells Robert he had the delivery men remove all of Robert's furniture and replace it with a brand-new living room. He invites Robert to attend a university lecture that evening, where he shows the class a video recording of his vandalism of Robert's flat, insisting it constitutes “performance art” and that he will email the clip to all the students. After the lecture, Sandy claims the living room now looks nothing like the video, implying Robert has nothing left to use as blackmail. He tells Robert to stay away and reveals his disdain for the man.
Robert breaks into Sandy's house and ultimately finds the original handwritten manuscript of A Patch of Fog, revealed to have been written by his own father. The shocking truth dawns: Sandy has been a fraud his entire life. When Robert resists handing over the manuscript, Sandy accuses him and a struggle ends with Robert dead—the result of Sandy losing his grip during a headlock.
Sandy dumps Robert's body into a lake and drives away with the idea of starting anew. But a rope becomes wrapped around Sandy's ankle, pulling him overboard as well; he is dragged beneath the surface, drowning alongside Robert.
