The protagonist of this film is Chance, the gardener. Middle-aged and mentally challenged, he has lived in a lovely little mansion all his life, never allowed outside of it by orders of his benefactor, "the old man". Chance contentedly spends his days gardening and watching TV; he learned his manner of speech and how to interact with others from the latter (often imitating the gestures people make as he watches the shows). He is allowed to wear the no-longer-used suits of the old man, and cuts an elegant figure that betrays his simple mind.
When the old man dies, Chance is evicted and forced to enter the real world, taking little more with him than a few suits and his remote control. As it turns out, he lives in Washington, D.C., and when he's accidentally injured by a car, its passenger turns out to be Eve Rand, the wife of banker/political kingmaker Benjamin Rand. Since Ben is dying, a hospital facility has been set up at their grand estate and Chance is invited to recuperate there. When he is asked his name, Chance says he is "Chance the gardener" but is misunderstood and from that point on is called "Chauncey Gardiner" by all the other characters. And when he tells them that he was ousted from his garden by lawyers, they think he is speaking metaphorically - that's he's really a ruined businessman. Ben is impressed by Chance's calm nature and the next day allows him to sit in on a meeting between him and the President of the United States. When they ask his opinion on the economy, all Chance can answer with is a comment on the passage of the seasons in the garden - which is taken as a comment on economic turnarounds. From this point on, Chance's star rises higher and higher - he is invited to talk shows and receptions, and the FBI and CIA try to suss out his past (impossible, as Chance's isolation means he never even had a birth certificate to be traced). Before he dies, Ben makes Chance his heir apparent and possible presidental candidate. Ben also instructs him to take care of Eve, who has grown fascinated with Chance. In fact, only one character in these power circles, Ben's doctor, figures out who "Chauncey" is, but doesn't reveal it to others because he realizes that Chance is not being deceptive, but is unknowingly mirroring the needs and wants of everyone around him. Chance, through all this, never really changes save for his friendship with Ben and blossoming love for Eve. It is his innocence that gives him the ability to walk on water in that final scene - because he doesn't know it isn't possible to do that. (I must give credit where credit is due; I learned this interpretation of the ending from this website that goes into why and how it was shot.http://www.disinfotainmenttoday.com/darenet/lastshot.htm)