The Machine picks out the social security number of the detective who is interested in Reese and Finch’s work, Joss Carter. In a flashback, she is seen in her previous incarnation as an army interrogation officer getting the better of an arms supplier with strong links to al-Qaida. Back in the present, two men discuss how to kill her.
While keeping her under close surveillance, Reese and Finch narrow down the list of possible perpetrators to three people: the first is the man who organised the killing of an informant called Ronny Middleton in a drive-by shooting, thought to have been ordered by a Mexican gun runner Carter was investigating; the second is a man called Kovach who is the husband of a woman who has made several complaints to Carter about domestic abuse; the third is Carl Elias, the notorious, illegitimate son of a Mafia crime boss who is determined to retake control of the streets from the Russian mob.
Who is trying to kill Carter? Is Reese close enough to keep her safe? What past actions does somebody want her to pay for?
Reese and Finch go into overtime when The Machine gives them four persons of interest.