A serial killer who thinks of himself as The Watcher murders a sex worker, Mona, in order to cut a retinal implant out of her. Soon after, private eye Ruth Law is hired by Mona's mother to investigate the murder, which the police write off as a gang killing. Ruth uses a number of implants to strengthen her body, and is addicted to her Regulator, which tamps down her emotions. She must switch it off once per day to allow her brain recovery time. During this time, she is overwhelmed by grief over the loss of her daughter, who died when Ruth was unable to take a shot at the man who had taken her daughter hostage. While the Regulator prevents Ruth from making decisions motivated by emotion, she takes the job in part out of empathy for Mona's grieving mother. Meanwhile, the Watcher extracts footage of Mona's clients from her retinal implant, and is able to use screenshots to identify one of her clients as a prominent Chinese official. He releases the footage, creating a scandal that gets the official fired, then uses the infamy of this move as proof that he can ruin people as he begins to blackmail other highly-placed former clients of Mona's. Ruth's investigation continues, and she's able to rule out gang involvement. After inspecting Mona's apartment, she meets a neighbor who runs a household wifi device that inadvertently scans the movement of people through walls. She obtains the device and hires an MIT student to retrieve a low-resolution replay of Ruth's murder, giving her more information about the killer, then contacts her ex-husband, the chief of police, and convinces him to let her use the NCIC database to narrow down other potential victims of the killer. When the footage of the Chinese official hits the news, revealing Mona's face, Ruth realises it was recorded from a retinal implant, and this revelation leads her to figure out the identity of his other victims, and the likely identity of his next victim. Ruth reaches out to the girl, Carrie, and learns that she has a new client coming that night, so Ruth hides in Carrie's house waiting for the Watcher to strike. When he does, she attacks him, but he triggers a small EMP that disables Ruth's implants, including her Regulator. He takes Carrie hostage, and Ruth is faced with the same scenario that led to the death of her daughter. This time, she doesn't hesitate to take the shot, and while she does clip Carrie, she hits the Watcher in the knee, taking him out of action. She cradles Carrie as they wait for the police to arrive.