The initial suspect in a grisly murder, Simon Doyle (Joshua Gomez), claims to be a time traveler from the future who has come back to prevent a disaster. Castle is intrigued while Beckett is dismissive, but their investigation eventually leads them to the true killer, and events begin to play out eerily similar to Simon's stories.
- Ambiguous Situation: By the end of the episode, it's not made clear whether or not Doyle and Ward are from the future.
- The Chosen One: Doyle reveals that the killer's target, Paul Deschiles, would save mankind from the energy wars by creating an energy shield that would disable all the fascists' weapons.
- Contrived Coincidence: If Doyle is not really from the future, the episode is full of this.
- The Dreaded:
- Garrett Ward terrifies everyone in prison after he was arrested.
- Doyle is this to Ward, who fled just after seeing him.
- The End... Or Is It?: The killer is caught, but Ward's smile suggests otherwise; that the war has just begun.
- Everything Sensor: Downplayed. Doyle uses this to track down those who had used time travel as there are still some traces of it. Oddly, the NYPD techs weren't impressed by it, referring to it as some junk, but it could be that it was deliberately made from period-accurate tech in the event that it was lost.
- Foreshadowing: All of Doyle's claims were preludes to what happens in the last two seasons: Beckett is offered the chance to run for New York State Senate, and she and Castle have three children in the Grand Finale.
- Make Wrong What Once Went Right: Castle and Doyle believe that Ward went back in time to kill Paul Deschiles, allowing the fascists to win the energy wars.
- Oh, Crap!: Upon learning who "the child" is, Doyle panics and even after getting sent to lockup, he calls for Beckett and Castle to protect Deschiles from getting killed.
- Real After All: Possibly. Doyle's warnings about the future have some scary credibility but we are left unsure.
- Revenge: It's presumed Ward was after Paul due to the latter uncovering his bomb plot years ago. Whether that's true or not is unclear.
- Shout-Out: References to The Terminator and 12 Monkeys were made.
- Stable Time Loop: After the case has been closed, Beckett spills coffee over a letter. She picks it up, and realises it now looks exactly like the photo of the letter Paul had in his possession.
- Amusingly, the non-time travel explanation has something akin to the boostrap paradox. If Ward is really not from the future, then his actions make no sense, even if you buy the assumption that he is crazy, as he is operating off of a letter than he would have had no means of getting because he doesn't yet how to find the person who has the original.
- Stealth Hi/Bye: Doyle has a tendency to disappear just when the detectives wanted answers, furthering the assumptions that he is from the future.
- Ungrateful Bastard: Despite Doyle driving Ward away, Beckett still arrests him.
- Whole-Plot Reference: The time travel story, whether real or not, is basically The Terminator minus the robots, which Esposito lampshades.
- You Can't Fight Fate: Doyle explains that small changes in the timeline won't matter since the time stream would auto-correct it. If the change was big, well...
