Smart fills in for the endangered King of Coronia, his exact double.
Interesting episode, adds some romance to the formula just when the series needed an injection of it. I have to admit I'm a sucker for "travel" tales in many shows, and this one allows Don Adams to show off impressive impersonation skills (his Ronald Coleman accent is suave yet hysterical). Its a little predicatable in a "Prisoner of Zenda" kind-of-way, but the plot of the real king's evil brother is funny and he's inept, always a good combination. Nice sword play at the end of this episode.
The reason "Get Smart" seems to have life for four good seasons (and one pretty mediocre one) is its ability to keep reaching for popular culture to lampoon - so even as it sometimes relies too much on the same "catch phrases", the subject matter often stays fresh.