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.

Stakes rise as the 12th Precinct matches wits with serial killers Jerry Tyson (3XK) and Dr. Kelly Nieman. With the clock ticking, Castle and the team must crack the case before the killers claim their next victim.