Supernatural -6×15- The French Mistake

Raphael launches an attack on Castiel and his allies, sending a particularly frightening angelic hitman after Balthazar, Sam, and Dean. In an effort to protect the brothers, Balthazar sends them to an alternate universe where they are the stars of a TV show called Supernatural and they are actors named “Jensen Ackles” and “Jared Padalecki.” The brothers are confused when Castiel appears to be a tweet-happy actor named Misha Collins and Sam is married to Ruby.

I've never watched this show but this episode seems so special and original that I've watched and it's good and funny.