Toby Nealey (George MacKay) is a politically-minded, 23-year-old graffiti artist who breaks into the homes of upper-class individuals and leaves behind the message "I CAME BY" on the walls. His latest target is retired judge Hector Blake (Hugh Bonneville). Despite the judge appearing socially progressive in public (he would take on cases pro-Bono on behalf of immigrants), Toby believes he's a hypocrite (Blake's father was a colonialist and Blake still has ivory pieces of antiques in his house). When Toby enters Blake's house, he discovers a pottery studio and an imprisoned man in the basement. But Toby has to leave when Blake returns unexpected (he had received an alarm about his wireless alarm system being disabled at his house).Toby tries to tell his friend and former accomplice, Jameel "Jay" Agassi (Percelle Ascott), about what he found. However, Jay's distracted by personal troubles with his pregnant girlfriend, Naserine "Naz" Raheem (Varada Sethu). Naz is thrown out of her house by her parents and Jay has to take her in and provide for her. Jay says he cant do anymore missions with Toby. He refuses to listen to Toby, when he seeks help with what he saw at Blake's house. Toby anonymously calls the police, but they find nothing in Blake's house and leave. There are noises coming from the basement, but Blake makes it look like they are coming from the washing machine.Lizzie (Kelly Macdonald), Toby's widowed mother, has grown increasingly worried about her son's lack of direction. When she and Toby get into a shouting match, she throws him out of the house. Toby insults Lizzie by saying that his dad bought the house and she has done nothing to earn it. To top it, he demands that he be given the money that his dad left him, but Lizzie says he is not ready to handle that kind of cash. Toby returns to Blake's residence to free the prisoner, but Blake again returns unexpectedly & murders Toby with a cricket bat. He then uses the kiln in his studio to cremate Toby's body, and flushes the ashes down the toilet.Jay is reluctant to disclose his graffiti activities with Toby to the police or to Lizzie, but plants a letter addressed to Blake in Toby's room. The letter spurs the police to visit Blake's home again (Since the last known location of Toby's mobile was also the same street as Blake's house) and they find the basement prison, which Blake claims is a "panic room." Blake is arrested for obstructing the investigation, but uses his connections to quickly get released.Lizzie begins following Blake. She witnesses him bring a man to his house: Omid (Yazdan Qafouri), a gay Iranian asylum seeker. Omid works at a spa that Blake visits and meets him there. Blake has offered to help him get permanent residency in the UK. Blake recalls how his cruel father took an Indo-Persian man as a lover, and this led to Blake's distraught mother's suicide. The teenage Blake eventually assaulted and nearly killed his father's lover. Despite being drugged, Omid manages to escape (Lizzie was lying in wait outside Blake's house and sees everything. Blake doesn't pursue Omid due to fear of being recognized by Lizzie). However, Omid won't go to the police due to his precarious residential status. The next day, Blake tracks down, threatens, imprisons, and murders Omid. On Omid's phone, Blake sees many messages from Lizzie and knows that Lizzie is Toby's mother.Increasingly desperate, Lizzie asks Jay to help her break into Blake's house. Jay refuses because he's a black man with prior charges, and is responsible for his new-born son. Lizzie tells Jay that she knows he and Toby were the graffiti painters by the tag of "I Came by".. She had found a USB of Toby where he stored photos of all his jobs and found Jay's photos in there as well. Lizzie enters Blake's house alone, only to be caught, murdered, and cremated. When Jay goes looking for Lizzie, he realizes she's dead and blames himself. Jay and Naz's relationship falls apart due to his lack of communication and hiding the past from her. Sometime later, Naz informs Jay that Blake is going to attend the Birlstone School's tercentenary (300th) anniversary celebration.After Blake leaves the event, Jay follows him to a country estate. Jay fights and subdues Blake. Jay then goes to the pottery studio in the estate's garage and finds a prisoner: the same man that Toby originally discovered. Jay calls the police, frees the prisoner, and flees the scene. The police arrive and find Blake bloodied and restrained in the house. On the wall, Jay has left behind graffiti reading "I CAME BY."