When a beautiful woman is found dead in an alley, all clues point to her being a victim of the notorious Triple Killer. The Triple Killer, or 3XK, terrorized New York City four years ago, then mysteriously disappeared. Now he's back. In a deadly battle of wits, Castle and Beckett race against time to stop 3XK before he claims his next victims. Lee Tergesen of "Oz" and Michael Mosley of "Scrubs" guest star in "3XK," one of the most thrilling episodes of the series to date.