I don't get to watch Castle as regularly as some shows, but I have watched enough to know that for a chunk of the season, Beckett has been dating fellow cop Tom Demming (Michael Trucco), and in last night's finale, Castle seemed to realize the relationship was getting serious. Yes, there was a main plot involving a spy game, but I cared more about the Castle/Beckett relationship.
So Castle is past deadline on his book, and his writer friends (Cannell, Connelly, and Patterson) suggest that perhaps Beckett has been more a distraction than a muse. Castle considers backing off his observer role, letting Beckett work more cases without him.
While none of the cops say they will miss Castle. shots of his usual chair sitting empty give us a sense of what his absence will mean. The cops also tell Beckett that, by now, Castle has done enough research to write a dozen books. He's sticking around because...
So Beckett decides to break up with Demming and is ready to take Castle up on his Memorial weekend invitation to the Hamptons. The problem: By this time, Castle has decided to take his agent/ex-wife with him so he can finish his book. He'll be gone all summer.
And, significantly, as Castle walks off, Beckett calls out hopefully, "See you in the fall?"
The plots and police procedure on Castle don't hold together that well, but the show has always had chemistry. This episode proves they know what to do with it.