Shiptease: Castle and Beckett pretending they're married. staring at each other when he said he hasn't found the right woman yet.
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 Castle know that ultimately, real life detective cases are not like the ones in his books, all neatly wrapped where the hero brings in the unsympathetic killers to justice.
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.