Mark Bruner was a serial killer responsible for the murders of at least 17 teenage girls in New York before he was caught, tried, convicted and executed.
In the opening of the episode two men find a girl beaten and strangled to death. A strange pattern is carved in the girl's skin, and the police find similar patterns carved into two victims in old, unsolved cases. The most recent victim was last seen leaving a bar, where a waitress recalls a strange behaving man leaving right after the girl.
Briscoe and Green meet up with the detective from one of the other unsolved murder cases, and he gives them a sketch of a man who was seen at the same library as the victim was last spotted. The detectives show the sketch to the waitress, who confirms that it's the same man she saw at the bar the night of the most recent murder.
After a number of fruitless interviews with the victims' families, Briscoe reasons that the killer could be a cab driver, since all victims were last seen far away from their homes. Briscoe and Green start going through the cab drivers and stumble on to Mark Bruner, who lives in a dark apartment with no telephone or TV. Bruner behaves oddly and elusively to the detective's questions and they bring him in. He is recognized in a line up by the waitress and arrested.
In court, Bruner's court appointed lawyer, Jessica Sheets, doesn't ask for bail, even though he pleads not guilty. She later motions to be removed from the case, revealing to McCoy that she is terrified of Bruner. It is indicated that he has threatened her. Bruner gets a new lawyer from Legal Aid, Tim Schwimmer. During a meeting with Schwimmer, McCoy and ADA Southerlyn, Bruner casually admits to killing the girl and that he has at least 15 other victims stashed up at a secret location. He also reveals - much to his lawyer's dismay - that he gave the location to Schwimmer who went there and checked.
Bruner appears ambivalent to the threat of the death penalty, which he considers no worse than spending the rest of his life in jail. He confesses to all murders and is executed. Most of the bodies, however, were never found.