Jack Wood is the nephew, press secretary and heir apparent to Mayor Fred Donovan whose Democratic political machine has run the Midwestern town of Palmer City for a quarter century. Donovan is aging, and the corruption in the city is getting out of hand. When Jack suffers a head injury that leaves him with a mysterious side effect -- he has an overwhelming compulsion to bluntly speak his mind -- he is soon revealing scandals at press conferences and is fired by his furious uncle.
The Republicans, desperate to find any candidate to run against Donovan, offer their nomination to Jack knowing full well he has no chance of winning. They simply want to embarrass the mayor by using his oddball nephew against him. It works initially, as Jack's early efforts are met with derision. But as his condition slowly comes to light, the voters realize they have something special on their hands-- an honest politician, one who cannot tell a lie -- and his campaign catches fire.
Jack's psychiatrist, the eminent Dr. Margaret Benson, is Donovan's longtime nemesis, and the mayor suspects she is behind his nephew in a cynical plot. He sets out to prove that Jack is lying about not being able to tell a lie and soon even Jack's lover Melinda Wheeler begins to question his honesty. The campaign trail is strewn with booby traps as the "real truth" keeps emerging, and then changing, and then emerging once again right up to election day. And beyond.