Above the Influence: Melanie's addict ex turned out to be this, as when she tried to leave her family for him, he turned her down knowing he was in no fit state to be in a relationship, and checked into rehab in the hope of cleaning up and being worthy of her in the future.
All for Nothing: Sam killing Melanie was ultimately this, since while his crime was Not Proven, his lover ended up breaking things off out of fear.
Breather Episode: Between the brief chase of a murder suspect in the previous episode, and the gun battle in the next, this episode concerns a crime that took place years ago, with Castle and Beckett following the trail with little to no action to worry about in the present.
Constructive Body Disposal: The episode opens with a body being found in a construction site after someone attempted to dump it in the wet concrete but missed.
A Deadly Affair: Sam and Melanie were both having affairs, which was why she was murdered.
Castle and Beckett scrub up for the scene with Lanie in the morgue.
Castle scoffs at Ryan's suggestion missing persons are sometimes alien abductees, saying everything has a rational explanation. Later episodes will seemingly have him become the main Mulder of the group, although at one point he admits to Ryan that this is just to annoy Beckett.
Guilt-Ridden Accomplice: Sam's best friend, who he coerced into helping him dispose of Melanie's body.
It's Personal: The case becomes this for Beckett because there are some uncomfortable similarities between law enforcement's treatment of this case, and that of her mother's unsolved murder.
Literal Metaphor: Upon seeing that the body was frozen solid before it was dumped, Castle quips that this is "My first cold case." Then the team identifies the body, and find that she was reported missing five years ago—so this actually is a cold case.
Missing Mom: Beckett's mother was murdered, as was Melanie.
Papa Wolf: Melanie's father, a retired cop, went after her killer.
Pun: Castle when they first see the body. "My first cold case."
And when they learn that her husband murdered her and kept her body in a freezer for five years. "You can say it. That's cold."
Raised by Grandparents: Melanie and Sam's children have gone into the care of the Davidsons (Melanie's parents).
Red Herring: Melanie's boyfriend turns out to be a dead end; he was in rehab when Melanie died, so he couldn't have killed her.
Runaway Bride: Melanie. Castle read from the case file that she did this multiple times.
Screw This, I'm Out of Here!: Sam's lover left him after Melanie's disappearance due to him previously asking her what would happen if his wife "wasn't in the picture anymore". He didn't take this well, causing her to fear for her life to the point that she transferred at work to get away from him.
Ship Tease: Castle and Beckett staring at each other when he says he hasn't found the right woman yet. Later on, them trying to get into Sam and Melanie's headspaces on the night of the latter's murder.
There Are No Coincidences: There is a connection between Sam and Melanie's deaths: his murder was carried out in revenge for hers.
This Is Reality: Beckett lets a dissatisfied Castle know that ultimately, real-life detective cases are not like the ones in his books, all neatly wrapped where the hero either brings in the unsympathetic killers to justice or lets the sympathetic ones go free.
Title Drop: While theorizing with the detectives, Castle comes up with the name of the episode as a title for a hypothetical book based on the murder.
Tragic Keepsake: Castle guesses that Beckett's watch is this, for her father. It's not; her father is very much alive, going on five years sober. The actual keepsake is the wedding ring on Beckett's necklace, which was her late mother's.
"I never said it was five years ago." Beckett and Castle were assuming that a witness had an encounter with the police five years before. This line revealed a much more recent encounter and hinted that the police officer mentioned was an imposter.
Also "It was my mother", revealing Beckett's mother had been killed and beginning the long-running arc of her case being re-investigated.
Beckett calls out the detective who first investigated Melanie's case for not caring enough to find out the truth. Even Castle is mildly disgusted enough by the man to say, "If I ever go missing, I hope he's not assigned to find me."
When interrogating Sam's former lover, Beckett wonders why she didn't come forward with her suspicions years ago, which could have solved Melanie's murder. The woman justifies that she wanted a clean second chance and it doesn't matter now since Sam and Melanie are both dead, but Beckett and Castle clearly disagree.
