Kevin Kline portrays "Dave," a regular person asked to stand in for the President of the United States while the real president takes some time off for personal pursuits. The plan is straightforward: appear, smile, wave, remain silent, and then depart in the vehicle provided. That’s the idea, at least. When the actual president's activities lead to a coma, "Dave" finds his temporary assignment extended within the Oval Office. It appears that the First Couple have drifted apart, prompting the highly driven Chief of Staff, “Bob” (Frank Langella), to believe he can position himself for power if he can maintain this secret long enough. However, the First Lady, “Ellen” (Sigourney Weaver), immediately notices a change in her now affable, intelligent, and engaging husband. The premise evokes "Man in the Iron Mask," but here, the disguise is one of public image as she and "Dave" begin to see the possibilities for themselves and their nation arising from this unusual circumstance. As is his tendency, Langella dominates every scene he’s in—particularly when the storyline becomes more complex and his mental state comes into humorous doubt—and there are also several amusing moments featuring Ben Kingsley as the unfortunate VP “Nance” and Ving Rhames as his direct bodyguard. Weaver and Kline have a lively rapport throughout the film, and by its conclusion, you’ll undoubtedly be hoping for the best outcome for this ordinary man.

Source on TMDB