When a woman's dead body shows signs of an old killer's handiwork, Castle and Beckett discover that the Triple Killer, a murderer who vanished four years previously, may have returned. Meanwhile, Castle alternately encourages and discourages Alexis' involvement with a secret admirer.
This episode shows examples of...
- Felony Misdemeanor: When Beckett says that Castle has a menial job, like 3XK, he says that "Just for that, I base my next book on Esposito", and she looks hurt.
- Frame-Up: Jerry Tyson's plan for getting out of jail was to pay off his cellmate to commit new murders and then cooperate with the police to be let out.
- O.O.C. Is Serious Business:
- The first victim of the investigation wasn't killed in her apartment as per the killer's M.O. but this is a plot point: the vic had previously called the 3XK hotline four years ago that pointed to him.
- Castle tells his mother "I love you" to warn her he's been caught by a serial killer. It works. Martha tells Beckett and the cops roll up.
- Rule of Three: The reason Jerry Tyson is called the Triple Killer. He would kill three women in a timeframe of a week, go quiet for a month, and then kill three more.
- Serial Killer: The titular Triple Killer or 3XK is the main focus of the episode, noted to have dropped six bodies before going quiet for four years.