Like all the other reviews, I must agree about this being one of the best episodes. In fact, it got me to visit and get one of the seven Nikki Heat by Richard Castle books. Just something about how Richard Castle writes (verses the TV series), which reminds me of this episode.
As for the ending, the writers missed it, and SHOULD / MUST revisit this before the series end.
SPOILER, but given how long ago, I feel safe talking about it now.
The episode ends with Beckett and Castle walking off agreeing the Blue Butterfly was paste, but what if there were two of them. There are two reasons for two of them:
1) Many people have copies made to protect the original. Given how much protection the owner gave it, well, would he do the same for paste?
2) After the original went missing, maybe the owner had the fake made because he couldn't bear to have people know it wasn't there. Yet, the strain of worrying about the original is what took his life (heart attack). People didn't put two plus two together, because they thought . . .
Remember: Stan came up from the office with the Blue Butterfly. He didn't come from the alleyway with it (from behind the brick). Also the Blue Butterfly the police tested came from the murderer's home, which is the one Stan found in the office.
As for Joe and Vera, they (most likely) thought the owner (Tom D) found it in the alleyway, but in reality he had the copy made. As they were hiding, they wouldn't know about the copy.
So what about the future? Well, there are many ways to lead the viewers back to this story, such as a body . one of the guys who guarded Vera OR another treasurer hunter who felt Beckett and Castle got it wrong). Hmmm, his death and her involvement however slight MIGHT make her running for office difficult, thus they (Beckett and Castle) become involved. I put it this way, as how knows where the series is going with her maybe going into politics (AT THIS TIME). I do believe she will given the time travel episode with the coffee stain.
Anyway, this discussion is yet additional proof about the greatness of this episode. As one sign of a great story is when the reader keeps taking, thinking, and imagining / exploring its past and/or future.
Only one last thing to add to this review. A big and public thank you to the writers, producers, and actors for an enjoyable hour (well 40 minutes or so) of television! THANK YOU!! :-)