This Monday at 10/9c, Castle resumes its uninterrupted final run of episodes with the “great gift” of a flashback-filled trip down memory lane. But beyond that don’t-call-it-a-clip-show — and in light of the events of last episode (which was supposed to air first) — the road ahead for Rick and Kate looks a bit bumpy. Series creator Andrew W. Marlowe spoke with TVLine about putting the popular pairing to the test, culminating in a top-secret finale where they and their relationship either pass… or fail.

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TVLINE | What story were you wanting to tell with these last few episodes, leading into the finale?

The last couple episodes are about how [Rick and Kate] have been in a relationship for about a year now, and that’s about the time people start asking questions, about where things stand. They’ve been living in this fun bubble, but there comes a time where real life intrudes and things come in from the outside and force the characters to ask questions about where they’re going. By the time we get to “The Human Factor” (airing May 6), those questions will start aiming us towards some complications that come in our final episode, challenges to the relationship that both characters are going to have to confront.

TVLINE | With “The Squab & the Quail” airing ahead of schedule last week, what did viewers miss out on not having seen “Still” first?

Not too much – and I will let them discover that. But there are a couple of really important relationship moments and serious moments in “Still.” As you know, that episode, the 24th episode, was added very late in the game, so we took a very particular point of view on how we were going to get it done. And it turned out, I think, in spectacular fashion. It’s a great gift to our loyal fans. There are a couple of moments in there that they have to look forward to, that they’ll really enjoy.

TVLINE | Would you be loath to call it a “clip show”?

I would. We certainly have clips in it, but…. When I realized we only had a couple of days to shoot it and we’d have to fill it out with clips, I set out to make the best “clip show” that television has seen, and I think our team pulled it off. It’s a great episode, because we use the clips to forward the storytelling. It is a great trip down memory lane, but we also have some high stakes and interesting stuff going on in the main story.

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TVLINE | Circling back to “Squab” for a minute: In your mind, what all informed Kate’s ever-so-slightly wishy-washy response to Erik Vaughn asking about her relationship status?

Well, I think what has dawned on her is [she and Rick] haven’t really talked about it. Does Castle think the relationship is serious? Does she think it’s serious, where they’re headed? They’ve been in it for a year and they seem to be having a really good time and having good moments, but…. This a guy who’s been married twice, she’s not getting any younger, she’s thinking about her future… and does her future include him? And is that part of his plan?

It’s only human to ask those questions at this point. I don’t think it signals that there’s anything “wrong” with the Castle/Beckett relationship, but I do think that she wonders what comes next — and as we get towards the end of the season, Castle himself is going to wonder what comes next. As a guy, he is somebody who doesn’t rush into commitment if things are working well the way they are. But Kate has a career and she’s got her next steps to think about. When an Internet billionaire is making a play for you, if she was single, I think we know what she would have done in [last week’s] situation. It speaks to her level of commitment to Castle [that she pushed Vaughn away], but it also speaks to a desire to know what happens next.

TVLINE | Aside from snipping symbolic videogame cords and offering candlelit massages, will we see other instances of Castle trying to course-correct?

As we head into “The Human Factor,” there are other, more professional issues that start to arise for the Beckett character, that begin to complicate things. We have that to look forward to as we come to our season finale, which is an emotional season finale. But instead of going into Beckett’s mother’s mythology like we have the last two seasons, and doing something operatic, I wanted to really focus on where Castle and Beckett were. So we have a great case but it kind of takes a back seat to wrestling with the bigger life issues that both of them are in the midst of.

TVLINE | “Human Factor” guest star Carlos Bernard (24), like Ioan Gruffudd, is not an unattractive man. Does his character serve a purpose similar to Erik Vaughn?

No, he serves a different purpose. He’s a complicated character, but he opens Beckett’s eyes to a greater world out there.

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TVLINE | Talk about the title of this season’s finale, “Watershed” (airing May 13).

Every so often in life you face a watershed moment, and I have to give props to [producer-wife] Terri Miller for coming up with the title. It was absolutely perfect — especially since in our A-case we find a body in a water tank. It was a great title that spoke to both the personal and the professional aspect of the storytelling.

TVLINE | Are you leaving viewers with a cliffhanger?

There is a bit of a cliffhanger, yeah. It’s an emotional cliffhanger that I think will propel us into some great, complicated, interesting storytelling as we head into next season — assuming the TV gods are kind and grant us one.

TVLINE | [Grey’s Anatomy creator] Shonda Rhimes is never shy about saying she scrapped a finale plan late in the game and reworked it from scratch. When did it become clear to you how you wanted to end Season 5?

Every season you have a great map, but it’s like driving through fog — the closer you get, the clearer it gets. So about seven or eight episodes out, I knew, ballpark, where I wanted to land. And part of it was informed by some really big storytelling we had in the spring, dealing with [Senator] Bracken, the two-parter [about Alexis’ kidnapping], what we’re doing in “Still”…. And I wanted a quieter, more emotional finale, without the fireworks. But you do go back and forth. We were fortunate that it [comes at] that period of time where people start asking the hard questions about relationships, and in the finale something comes out of left field that both characters have to wrestle with, something that would be a challenge to any relationship. Having clarity seven or eight episodes out was very helpful, and we feel pretty good about where we landed.

Want more scoop on Castle, or for any other show? Email insideline@tvline.com and your question may be answered via Matt’s Inside Line.