Turk and Rooster, two veteran NYPD detectives and longtime partners, are hunting a serial killer who is murdering sociopathic criminals. They both have personal issues, and when they start working with a younger investigative team, Perez and Riley – tensions between the two teams is inevitable. Will they identify the possible connection between a recent murder and a case they believe they solved years ago; is there a serial killer on the loose, and did they perhaps put the wrong person behind bars?