The episode was just so amazingly executed, it covered everything, firstly it developed an already interesting mystery that is so personal for Beckett, it brought her and Castle closer together and yet kept their playful banter albeit at a minimum, and it highlighted how she thinks as much of him as he does of her, he is the only one she has shown the case wall to, not even her BF has seen it, and the one person she called to go to the meeting with her was Castle, they even called each other by their first names, a first in the series I believe.
Then there was Ryan and Esposito, in an episode like this they could have easily been sidelined, yet it developed both of them tremendously and had them in the thick of it, first having Beckett's back, then their part of the investigation that gave them an interrogation scene and then them sticking together under the torture of the hitman.
We even got to see another side of Martha in her scene with Castle, and then there's "The Kiss"....maybe not quite how some expected it to go down but it worked really well within the context of the episode, and despite both being able to play it off as just doing the job it mattered as seen by both of their reactions.
This was one of the most serious episodes of Castle and yet it kept some of it's humour, and rarely does an event type episode that is about a major overarching plot contain such a good balance, it managed to deliver a big dollop of forward motion on the shows one ongoing plot whilst developing the characters and their relationships a great deal, just fantastic stuff.