Rating: 10.46 million (2.7/7)

Regina Spektor's song "The Calculation" occurs just after Castle and Beckett discuss "the math" and "the equation" of the DB having a wife, a fiance, and two identities.

(looking at the DB's driver's license)
Beckett: All right, the name is Sam Parker, and the driver's license has his address placed in Connecticut.
Ryan: Maybe he's visiting someone in the building.
Castle: You mean dropping by...?

Beckett: Sam was pretending to be Jake before he met Sarah. I don't understand the fake identity.
Castle: Maybe to land the job.
Beckett: I understand changing your resume but changing your identity, the math doesn't quite add up for me.
Castle: You know what math doesn't add up for me?
Beckett: Hmm?
Castle: Two wives.
Beckett: One wife too many for you?
Castle: Two wives too many.
Beckett: Seems like the common denominator in that equation is... you.

(Alexis has just shown up looking very mature for her "field trip" to work as a volunteer at the police department.)
Castle: That ever happen to you with me...? One day you look and see your boy's all grown up?
Martha: Still waiting for that moment, actually...

Castle: Well?
Beckett: Alexis? She's fine.
Castle: You know, her first day at preschool, I hid outside in the bushes all day just to make sure she was all right.
Capt. Montgomery: First time my son went to summer camp I followed the bus all the way to the Adirondacks.
Beckett: You two are both either very sweet or very creepy.

Castle: Was Sarah Reed on the team of engineers that developed that technology?
Lance: Yeah, that's right.
Castle: That's probably why Sam Parker went out of his way to romance her. For access. It's pretty smart.
Lance: Pardon me, but, don't you mean 'despicable'?
Castle: The two aren't mutually exclusive.

Title: One Man's Treasure

Alluding to the adage, "One Man's Trash is Another Man's Treasure", in that something one man may have no use for could be very useful to someone else. This ties to the recycling element of the story's background, in the form of a company that got rich dealing with someone else's trash (toxic waste).

Martha: If that comes any closer you're going to have Blackhawk Down on your hands!

Blackhawk Down is a 2001 Ridley Scott film about a 1990s Somalian military mission (i.e., "based on a true story") that went seriously wrong and wound up with a downed helicopter and a large team of Army Rangers trapped by an ambush. The resulting national "black eye", and then-President Clinton's rather sheepish response, was cited by Osama Bin Laden as one reason he did not believe the USA would respond to the events of 911 with military force.

Castle: A killer who cleans up after himself. Very American Psycho.

American Psycho was a 2000 film starring Christian Bale based on a best seller by Brett Easton Ellis. The story is about a yuppie investment banker who is also a psychopath.