I'm usually not into the whole 'using ambiguity to lend credence to otherwise ridiculous possibles' like all that psychic and supernaturalism BS but this plot was put together quite well and enjoyed it apart from the Beckett being the one to make the stain on the letter -which was way too 'on the noes', would been better if some remake that the picture showed a stain that the original didn't have.
What I really didn't enjoy was another dose 'side story Alexis' -you know the ones where she does (or wants to do) something naive/dumb, Castle is like "oh noes!"; Alexis says she is >18 YO and that she is "smart", then everyone else puts in their anecdotal BS 2 cents in, Castle apologizes and she gets her way.
I mean seriously? what she is now is quite different from the early episodes which depicted her as witty, responsible beyond her age and actually intelligent rather than being called "smart" because of her good grades and everyone for some reason assuming that being good at memorizing text translates to all areas of life.