<i>Castle</i> Castle

For once, Richard Castle might not be the nerdiest person at a crime scene.

On Monday's Castle, Beckett (Stana Katic) and Castle (Nathan Fillion) investigate a murder that could be out of this world. "We have a couple [crew members] on the show ... who are from The X-Files, and so in a lot of ways this was our chance to do our Castle version of an X-Files episode," creator-executive producer Andrew W. Marlowe tells TVGuide.com.

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It all starts when the coroner determines that the victim died in a rare way. "We find a woman who's been killed due to explosive decompression," Marlowe says. "There are very few things that cause it, and one of those things is exposure in outer space. Of course, Castle sets off jokingly suggesting that maybe aliens were behind it."Oddly enough, each turn in the case lends some credence to Castle's theory. Beckett & Co. quickly meet the victim's former colleague, a super-nerd who has written books about alien abduction and leads a support group for survivors. (It's oddly fitting that this role is played by Lance Henriksen

, whose sci-fi bona fides include a starring role on X-Files creator Chris Carter's Millennium.) Things get even more eerie when a mysterious Men in Black-like federal agent played by musician Lyle Lovett begins nosing around."He's a man of great mystery," Marlowe says of Lovett's character. "He's asked, 'What government agency did you say you worked for?' And he goes, 'I didn't say,' and that's the end of that conversation."

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This is hardly the first time that Castle's theories have been a bit out there. Just this season — to which ABC just added two episodes after the show's most recent episode topped timeslot rival Hawaii Five-0 in total viewers (10.8 million vs. 10. 2 million) for the first time — Castle has posited that a psychic medium predicted her own death and than one victim killed by an antique bullet was killed by a time-traveling murderer. Even though it makes for fun scenes with Castle and Beckett, Marlowe says the ideas in Castle's adventurous mind are often the most difficult parts of the episode to write."A lot of times we know Castle is just tweaking Beckett, but we don't want him to come off as just clownish," Marlowe says. "What's a challenge for us is to find the outlandish theory that has some credibility, some portion of it that's grounded in truth. We often end up in a situation where his theory actually does pay off in the episode."

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Castle will be brought back to reality, however, when he is again reminded just how serious things are getting between his teenage daughter Alexis (Molly Quinn) and her boyfriend (Ken Bauman). "Castle has to meet the parents, which is kind of fun," Marlowe says. "He has to sit down and break bread with Ashley's parents. We'll see how that goes."Castle airs Mondays at 10/9c on ABC.