Adam Baldwin Previews His Castle Visit SPOILER

Rick Castle is about to get Slaughtered.

In this Monday’s episode of Castle (ABC, 10/9c), Nathan Fillion’s novelist will dive head first – for better or for worse – into a storyline with a rough, tough and gruff NYPD detective named Ethan Slaughter and played by Adam Baldwin.

And while the episode obviously offers the geeky thrill of a reunion between the onetime stars of Firefly (where Fillion played Capt. Mal Reynolds to Baldwin’s Jayne Cobb), it will also serve a larger purpose, as the ABC hit closes in on its “very emotional” Season 4 finale (airing May 7).

Still processing the discovery that Kate remembers her shooting and the aftermath, “Castle is trying to figure out how he feels about Beckett,” series creator Andrew W. Marlowe tells TVLine. “He feels likes he’s a bit uninspired by her because of all this stuff that’s going on, but he hasn’t articulated that yet. So when he sees this larger-than-life cop figure on the news, he thinks, ‘Maybe I just need a new muse?’”

And make no mistake, Slaughter as a decidedly different muse will amuse. “He takes Castle on quite the ride,” says Marlowe. “Maybe it’s the most exciting case of his life, or maybe it’s, ‘The grass is always greener.’ That’s the journey he goes on.”