The Long Island Serial Killer: A Mother's Hunt for Justice tells the story of Mari Gilbert's search for the truth behind what happened to her daughter Shannan, a sometime escort who disappeared after a 'date' on Long Island. After Shannan (Katharine Isabelle, Hannibal) fails to come home, her mother Mari (Kim Delaney) knows something is terribly wrong. Pleading to police to take her concerns seriously, she keeps pushing the police for answers. Mari's insistence that her daughter not be overlooked eventually leads to a horror hidden on Long Island for more than a decade 19 bodies of young women discovered buried in shallow graves along Ocean Parkway in the area of Jones Beach State Park.