I remember this because Will and I watched the episode recently (we're working our way through Castle).

This is the third episode to deal with the 3XK (Triple X killer). The first episode was in season 3 and they run across the case and realize it's the same as a pattern from 4 years ago. The team works to solve the crime but 3XK does things like use a double to screw with time lines. He actually has everyone believing that he's the former cellmate of 3XK and they have him in a safe house.

Castle and Ryan go to the safe house and it's there that Castle figures out this guy Jerry is really the 3XK. Except he blurts it out and Jerry gets the drop on them and ties them up -- at some point Martha calls and Jerry gives the phone to Castle who says he's fine blah blah and ends the call with "I love you Mom". (or just I love you).

3XK does the evil villain thing and is either about to kill them when Beckett and the police swarm in or has left them.

Martha knew something was "terrible wrong" when Castle said I love you so she called Becket who sent everyone to the safe house.

Then in a later episode in season 4 a woman is killed with Ryan's gun. They spend a lot of time trying to figure out how the killer got his gun from Jerry and eventually they get a lead from the guy who had the gun that the guy who sold it to them, Jerry was going off to some state. But Castle doesn't believe it because it's too easy.

That episode is the one where the girl is the tutor and it turns out she's secretly involved with the guy she's tutoring - one of the sons of a Yakuza boss. The older brother killed the girl, but the love interest brother went into witness protection after offering to give evidence against his family.

Castle and Ryan go to the safe house and it's there that Castle figures out this guy Jerry is really the 3XK. Except he blurts it out and Jerry gets the drop on them and ties them up

Thanks, askye! This is literally the only part I remember about the episode, but now that you describe it, I remember it much better, especially the "I love you" part.

If I didn't recognize the actor from Pan Am, I wouldn't remember the character at all either. As it is, I appreciate the recap!

If I didn't recognize the actor from Pan Am

That's why he looked familar!

ION has been playing Numb3rs repeats. I forgot how much I missed that show.

Have you seen Eppes the Younger in Partners?

ita ! I have. I'm not completely won over by the show yet, but I am hopeful. I'm afraid that the premise is going to get old very quickly.

He's really cute, though. When is he not?

He is, and I also like the actor from Ugly Betty, so I really want the show to be good.

When the previews indicated that Castle was going to be a suspect I fully expected it to be a frame job by 3XK since 3XK was obviously set up to return at some point (and might again!). I still gasped when Jerry showed up. I wasn't expecting him to saunter right into the precinct to taunt Castle!

Yeah that was surprising when he came in. And also I was glad that it was left open and they didn't try to be ambiguous about it with everyone thinking he was dead but no body.