Nathan Fillion will take a break from solving crimes on ABC's "Castle" to appear as himself in an upcoming episode of "The Big Bang Theory."
The "Firefly" alum will make a guest cameo in the Feb. 19 episode, according to Entertainment Weekly. Leonard (Johnny Galecki) and Raj (Kunal Nayyar) spot the ruggedly handsome actor at a restaurant and debate whether to ask him for a photo.
Fillion has won over geeks everywhere with his roles in "Firefly," as Captain Malcolm Reynolds, "Dr. Horrible's Sing-Along Blog," as Captain Hammer and "Buffy the Vampire Slayer."
The CBS comedy has always showed its adoration for the short-lived, cult series "Firefly." Summer Glau, who played River in "Firefly," made her own cameo appearance in season two of "The Big Bang Theory."
Sheldon (Jim Parsons) frequently curses the show's early cancellation (Fox pulled it after one season in 2003) and wrote a clause into his and Leonard's original roommate agreement that Friday nights would be reserved for watching the show because he believed it would be on for years.
This isn't the first time ABC has allowed Fillion to appear on a rival network. He also guest-starred on the NBC comedy "Community" last year.
"The Big Bang Theory" airs on Thursdays at 8 p.m. on CBS.
