In the Castle season 1 finale, "A Death in the Family" Kate Beckett is devastated when Castle does the one thing she asked him not to do, look into her mother's murder.
Added:
Alexis attends her first prom in this episode. That's enough to make any father cry.
Added:

Castle Season 1 Episode 10 Quotes

Beckett: (Clears throat) Ahem. What's the deal with men and boobs, anyway?
Castle: Biological. We can't help it.
Beckett: But doesn't it bother you that they're so obviously not real?
Castle: (Pauses) Santa's not real. We still love opening his presents.

Oh somebody's been watching to many Bruckhiemer Films.

Castle