Plot summary
John Reese, a former CIA agent and Army Special Forces soldier now a homeless alcoholic, is arrested for getting into a fight on a New York subway train. He is bailed out before homicide detective Joss Carter can question him regarding DNA evidence linking him to a string of killings. Reese is brought to mysterious billionaire Harold Finch, who explains that after the September 11 attacks, he designed an algorithm capable of predicting crimes by analyzing behavioral patterns in troves of surveillance data. The program ("the Machine") provides the social security number of a person it predicts to be involved in an impending crime, victim or perpetrator.
Finch enlists Reese in helping him thwart the crimes, appealing to Reese's sense of guilt following the death of his girlfriend Jessica. Reese's first person of interest, ADA Diane Hansen, oversees a ring of corrupt police officers and plans to kill a fellow DA. He blackmails one of the corrupt officers, detective Lionel Fusco, into being his inside source at the NYPD. Reese then reveals Hansen's corruption to an open court, kills corrupt detective Stills, and links Fusco to the crime to ensure his future cooperation.
Credits
Adapted from Wikipedia contributors, Person of Interest season 1. HTML formatting and reference markers removed.