'Castle' video sneak peek: Kate's shocking secret revealed!

Well, well, well. Who would've guessed that inside "Castle's" no-nonsense Kate Beckett (Stana Katic) beats the heart of a fangirl!

The homicide detective gets her geek on in Monday's (Nov. 5) new episode, "Final Frontier," astonishing her new boyfriend Rick (Nathan Fillion) with her extensive knowledge of the sci-fi show "Nebula-9."

PHOTO GALLERY: "Castle" visits the final frontier

Check out the preview of the pair questioning a suspect ("True Blood's" Christina Moore), and let us know: Could Kate be any more adorable this season?

"Castle's" "Final Frontier" -- directed by "Star Trek's Jonathan Frakes -- airs Monday at 10 p.m. on ABC.