With it's finale on the face of Mount Rushmore and the crop duster chase scene, this is one of Hitchcock's most iconic movies, probably along with Psycho with it's shower scene and Vertigo with that infamous climb up the Mission Tower to ultimate calamity. Who knew so much jeopardy could result from a (albeit less than innocent) case of mistaken identity. One of the things I love about this film is the fact that quite early on the 'un-named Government agency' that is running the undercover operation utilisiing Eva Marie Saint's character discuss the situation very civilly and then casually decide to let poor Roger twist in the wind in order to protect their agent in place and to see what happens!
What does happen is a frantic cross-country chase where the villains chase Roger, who is simultaneously chasing the man that they think he is, in order to set things straight. The macguffin that we're let in on by the above 'unnamed agency' is: the man doesn't exist! Even so, Roger O Thornhill or ROT as he refers to himself (the O stands for nothing, by the way) manages to stay one step ahead of the villains whilst smooching his way cross country with Saint's somewhat shady Eve Kendall.
Very few people could take such a simple conceit — mistaken identity — and 'successfully' and so entertainingly spin it out to encompass all of the things that happen in this film, but Hitchcock - a master of pacing and of building 'plausible' suspense is probably one of the few.
Another film who's ultimate wit and charm doesn't seem to be dulled by repeat viewings, much like, His Girl Friday (previously mentioned) or Charade (upcoming). This is undoubtedly one of Grant's most memorable roles.