"Dave" is a very enjoyable movie with a happy ending that is probably more improbable than the numerous other improbabilites that are essential to the plot. Basically, it is the story of a young man named Dave who has two very valuable talents. First, he loves helping people and is delightfully friendly and outgoing. Second, he bears an uncanny resemblance to the President of the United States. As we meet Dave early in the movie, he uses both talents in the free market to earn a living. The first he uses to help people find work, i. e. he runs an employment agency. The second he uses to do business promotionals suggesting that the President recommends his client's products or services.
It is his second talent that drives the plot, which is similar to "On the Double", "That Night in Rio", "On the Riviera" etc where an "ordinary Joe" is enlisted to impersonate a prominent person on a short term basis. Since Kevin Kline, the star of Dave, lacks the comedic talents of Danny Kaye, the story is more straight-forward, along the lines of "I was Monty's Double". Dave is initially hired to substitute only very briefly for the real President so he can get away to a tryst with a paramour. But while making love to her, he has a stroke, so the President's Chief of Staff enlists the aid of the President's speech writer and the Secret Service men who originally hired Dave to make the switch more permanent until the President recovers. Actually, the Chief of Staff is hoping to somehow arrange it so that he will be made President, but, of course, only the speech writer knows that. The stroke apparently is so serious that there is little hope that the President will, in fact, recover enough to resume office.
The story is driven by the fact that the real President is a real jerk. He cares about no one except himself, but uses all of his staff to take the blame for his selfishness, i. e. somehow he gets the blame shifted to his staff for vetoing all those worthy programs that Hollywood loves to support, e. g. homeless shelters, aid to the needy, arts subsidies, etc. Of course, the Chief of Staff is an even bigger jerk. He wants to veto everything except aid and subsidies to himself. It turns out that the President's philandering and his hard-hearted attitude toward homeless shelters have driven a wedge between him and his wife, who wishes to have nothing to do with him, which is, of course, an important assist in the plan to substitute Dave.
Of course, the outcome is predictable. Dave and the First Lady fall in love when she figures out that he is not the real jerk she has come to know and hate all these years. She discovers this when, on a publicity visit to a homeless shelter, Dave shows an uncharacteristic (for the real President) interest in both the children at the shelter and her. Her suspicions are confirmed when she confronts Dave in his shower, conveniently located, like all showers in modern Hollywood movies, in the center of the living room, and discovers that he is missing some important moles and other tell-tale birthmarks. She clinches her suspicion by trapping him into telling about an experience that the real President never had. They spend a pleasant evening on the town together, but go back to the White House, where things are rapidly coming to a head.
The thing that has brought things to a head is that Dave, the quintessential nice guy, is tired of vetoing bills for the homeless and, in addition, he is determined to push through, entirely on his own, a full-employment jobs bill. This upsets the Chief of Staff, who, quite rightly it must be admitted, remembers that Dave is not the real President, and, although painfully aware that he also is not, feels strongly that he should be, and is, therefore, incensed when Dave fires him.
In the end, Dave gets his full-employment jobs bill, although, it is the Vice-president, having taken over as President, who pushes it through for him. We last see Dave running for City Council, the place, we are informed earlier in the movie, where the Vice-president got his start in politics. He is still running an employment service, which tells us that his full-employment bill, although passed, did not work.
I mention that the happy ending is probably the most improbable part of the whole movie, not so lmuch because it is improbable that a former First-Lady would marry the proprieter of an employment center, as the fact that, if Dave is successful in his political aspirations, she will in all likelihood wind up married to as big a jerk as before. I doubt very much that the President she initially married was a big jerk when she married him. He became a jerk for the reason so cogently made by Lord Acton, i.e. power corrupts. Dave is a wonderful man precisely because he has talents that he uses to serve other people. He sees the possibility of political office as greatly extending his ability to serve because it invests him with power, but power corrupts. As soon as he gets power, he will cease to be the Dave we know and love. He may not become exactly like the President he temporairly replaced, but he will have problems, because power corrupts.
One of the things that Hollywood, and even the American people find very difficult to believe, is that as soon as we begin using the power of law to force people to do what we would like them to do, or even what we feel they should do, it is personally corrupting.