This episode concludes the investigation of a pair of arson-related homicides, led by detectives Pembleton and Bayliss, with their Arson Squad colleague Det.
Kellerman deduces that this fire has also been deliberately lit, using gasoline-soaked "trailers" (probably made from toilet paper) and from his observation of the body he theorises that the victim was probably killed elsewhere before the building was set alight.
While Howard and Munch vie with each other over the upcoming sergeant's exam, Bayliss and Pembleton discuss the duty of informing members of the public about the death of loved ones, on their way to speak to the girl's family.
When he returns to the squad room, Kellerman complains that his "chemist" lead has turned up nothing, but then Pembleton receives another phone call from his anonymous informant.
Entering the precinct of the abandoned building, Frank pays for information from a vagrant, who directs them to a homeless old woman, Mrs Rosen.
Frustrated, the detectives argue with each other over their inability to crack the case, but then Bayliss scores a breakthrough, finding a match for the elusive blue van in vehicular records, and they bring in the registered owner, Gavin Robb, a young chemistry teacher at a local high school.
Under questioning, Robb protests his innocence, explaining that his connection to Mrs Rosen is because he often drives around at night helping homeless people, but he is also forced to admit that Bonnie Nash had been a student in his class the previous year.
Caught offguard, Robb inadvertently replies, "I didn't know it was there", effectively admitting he had been at the warehouse on the night of the Nash's murder, and he then confesses to the girl's slaying.
Mike initially turns it down, worried that he is not up to the challenge, but after visiting his father at his dead-end job on production line of a local distillery, he returns to the station house to accept Giardello's offer.