Dave Kovic, portrayed by Kevin Kline, operates a temporary employment agency in Georgetown, Washington, D.C. On the side, he has a striking resemblance to President William Harrison "Bill" Mitchell, whom he impersonates for various appearances. Things take a turn when Mitchell's unscrupulous Chief of Staff, Frank Langella, employs Dave to cover up an embarrassing situation involving the President's affair with White House staffer Laura Linney.
When the real President Mitchell suffers a stroke during an intimate moment with Randi, he is left in a coma, and Alexander seizes the opportunity. Along with Communications Director Kevin Dunn, they con Dave into stepping into the President's shoes under the guise of saving the country from chaos, especially with Vice President Ben Kingsley supposedly being mentally unstable.
Only Dave's Secret Service bodyguard, Ving Rhames, and the medical team secretly caring for the actual President are privy to this deception, while the First Lady, Sigourney Weaver, is kept in the dark. Alexander's ulterior motive is to manipulate the presidency through Dave, all while framing Nance to further his ambitions.
As time goes on, Dave starts to carve out his own path, winning back public affection for Mitchell and confusing media pundits, all while dealing with the complexities of a strained marriage within the Mitchell household. With the First Lady's undercover support, Dave endeavors to revitalize programs like a homeless initiative that she values, using funds he creatively reallocates from the national budget. However, Ellen soon realizes the nature of the deception, leading to a pivotal moment where Dave must convince her to stay and continue the charade.
Upon discovering Dave's newfound independence, Alexander tries to intimidate him into submission, but Dave stands his ground, revealing the risks Alexander faces should the truth surface. As pressure mounts, he organizes a news conference to announce significant policy changes, signaling he won’t be a puppet any longer.
In a surprising twist, Dave learns the Vice President isn’t the frail man Alexander described after a candid discussion with him, which further uncovers Alexander's manipulation. As the scandal envelops the White House, Dave remains steadfast in advocating for his employment initiative, bonding with Nance over shared experiences.
During a critical joint session of Congress, Dave as "Mitchell" courageously confesses his role in the debacle but strategically reveals evidence implicating Alexander as the true culprit. In a dramatic turn, he feigns a stroke just as he prepares for a formal apology, thus swapping places with the still-comatose Mitchell before he tragically succumbs to his condition.
The public’s affection for Dave eventually transcends to the real President Mitchell, creating a wave of mourning after his passing. As Ben Kingsley steps up as the new President, he embraces the employment program initiated by Dave.
Fast forward five months, and Dave is in the midst of his own city council campaign. The former First Lady, now grieving her husband’s death, visits his office cautiously seeking a new beginning, culminating in a heartfelt kiss, while Ving Rhames proudly displays his support with a campaign badge for Dave, suggesting a promising future ahead.
