One of the first things that jumped out to me in this episode, having seen all the series more than once, is Castle's role reversal. While going through files with the team, Castle shoots down one fanciful theory after another. Raised by other team members. All of them would have sounded just right coming from classic Rick Castle's mouth. Yet he's the one shooting them all down. Obviously new shows take a while settling into their own "normal", and writers vary so character personalities can vary too. But this is pretty strange.
This entire episode has Castle displaying a more mature personality than we're used to, and we've already seen in earlier episodes. Sure, there are a couple of times Castle degenerates, but otherwise this is not classic Rick Castle.
Despite this temporary role reversal, this is an amazing episode. I loved the first exchange between Alexis and her dad. It's scenes like this that made this show great and this was one of the better ones.
There's another great scene where Beckett first come's to Castle's apartment in the middle of a laser tag battle between Alexis and her dad.
And the "pretend we are married" scene. Here and the previous scene is one of the earliest, if not the first, Beckett and Castle synergy scenes where they finish each other's sentences while building a theory. And it's fascinating for the viewer.
This episode also begins a couple of other things for the series. First, while there are ups and downs, Castle and Beckett are really starting to accept one another. Also, this is when we learn part of Beckett's backstory which echoes through the entire rest of the series up to the last episode. Finally, the very last scene begins a thread, a very subtle one, of something that frequently divides them and pops up again and again until season 8 episode 15. I expand on that in my review for that episode.