A wonderful, funny, odd, surreal, and sometimes very sad satirical fable.
Sellers plays a man who is a complete blank, seemingly mentally handicapped, having spent his whole life working as a gardener, his only real enjoyment being watching TV. Through a wonderfully creative series of coincidences, he soon has Washington's elite read into his simplistic pronouncements whatever they want to hear, and the next thing you know he's an advisor to the President, and winning over TV audiences.
This could have been a one-joke film, but the terrific writing and directing, and the simply magical performance by Peter Sellers make this deep, complex, and moving. There's great supporting work too by Shirley McLain and Melvyn Douglas. Jack Warden overdoes the doofus bit a little as the president, and there are a couple of moments where the jokes feel forced, but this is as funny and socially incisive a film as has been made in the last 30 years.