At the end of Castle's two-part arc, Season 4, Episode 16: "Linchpin," CIA agent turned Russian spy Sophia Turner dropped a bombshell on her former lover before she was shot.
"Your father will be very proud," she said, pointing a gun at Rick's head. "My father?" he asked quizzically.
"You don't think you gained special access to the CIA back then because of your charm. You really don't know, do you? Guess you never will."
To say our minds are still blown is an understatement, but at the same time we're scratching our heads skeptically. Sophia was a liar and a murderer. Could there be any truth to her claim? And if she was, in fact, telling the truth, would Castle launch his own, secret investigation into his father's life?
According to show creator Andrew Marlowe, Castle's definitely thinking about it, but he doesn't even know where to begin. That's not to mean he won't eventually stumble upon some leads. After all, Sophia Turner might not even be dead.
"We are planting seeds for further storytelling assuming that we get good news on Season 5," he told TVGuide.com.
"I haven't decided yet, but it is something I absolutely wanted to bring up [once more in Season 4]. He is going to pursue it, but it is not something that he is going to pursue immediately. I think that he is still wrestling with it."