Toby Nealey, portrayed by George MacKay, is a politically-driven graffiti artist at 23 who targets the opulent homes of the elite, always leaving behind the provocative message, "I CAME BY." His latest mission takes him to the residence of retired judge Hector Blake. Although Blake showcases a socially conscious image by taking on pro-bono cases for immigrants, Toby is convinced that he masks his hypocrisy—after all, his lineage includes a colonial past, and his home is cluttered with ivory antiques. During his intrusion, Toby stumbles upon a pottery studio and an imprisoned man confined in the basement. However, he is forced to flee when Blake unexpectedly returns, having been alerted to his alarm system being breached.
Desperate to share his discovery, Toby reaches out to his friend and former partner in crime, Jameel "Jay" Agassi, but finds Jay preoccupied with personal issues involving his pregnant girlfriend, Naserine "Naz" Raheem. Naz has been disowned by her parents, leading Jay to take on the responsibilities of her care, which makes him reluctant to assist Toby. When Toby contacts the police anonymously, they investigate but find nothing amiss in Blake's home; Blake cleverly disguises sounds from the basement as mere malfunctions from the washing machine.
Meanwhile, Toby's mother, Lizzie, has grown increasingly concerned about her son’s aimlessness. Their escalating tensions culminate in a heated argument, resulting in Toby's eviction from their home. In a moment of anger, he lashes out, reminding Lizzie that she did nothing to earn their house, which his father bought, and demands the money his father left him—a sum she believes he isn’t prepared to manage.
Returning to Blake's home to rescue the captive, Toby faces a grim fate when Blake arrives early, attacking him with a cricket bat and subsequently using his kiln to cremate Toby’s remains, disposing of the ashes down the toilet. Jay is hesitant to reveal their graffiti activities to the authorities or Lizzie but, ultimately, plants a letter addressed to Blake in Toby's room, leading the police to revisit Blake's residence. This time, the officers uncover the basement prison; Blake attempts to justify it as a mere "panic room," but is arrested for obstructing the investigation, yet he soon leverages his connections to secure his release.
Determined to seek justice for her son, Lizzie begins to follow Blake, witnessing him escorting a man named Omid, a gay Iranian asylum seeker, whom Blake has promised assistance in securing residency in the UK. As their encounter unfolds, Lizzie learns of Blake's traumatic past—his relationship with his father's lover leading to his mother's suicide and his subsequent violent outburst against that lover. Omid, fearing for his life, manages to escape, but Blake quickly retaliates, finding and brutally murdering him, even as he discovers Lizzie’s involvement as Toby's mother through Omid's phone.
In a desperate bid, Lizzie solicits Jay's help to infiltrate Blake's home, but he hesitates due to his criminal background and responsibilities as a new father. Yet, Lizzie knows the truth; she found evidence linking Jay to Toby's graffiti. Undeterred, she goes in alone, only to meet a tragic fate at Blake's hands, ultimately joining her son in death.
Faced with the weight of grief and guilt for not being there when it mattered, Jay's relationship with Naz deteriorates further. Time passes, and Jay learns Blake will be attending the Birlstone School's tercentenary celebration. In a moment of resolve, Jay follows Blake to a secluded estate and confronts him, leading to a desperate struggle. During the conflict, Jay discovers the same prisoner Toby had found earlier, releasing him before making his escape.
When the police arrive, they find Blake bound and battered, while on the wall remains Jay's final mark—"I CAME BY." This image serves as a haunting reminder of Toby's mission, the tragic chain of events forever etched in their lives.
