A naive and idealistic Jefferson Smith, leader of the Boy Rangers, is appointed on a whim to fill a vacancy in the U.S. Senate. His plans quickly collide with political corruption, but he doesn't back down. Upon arrival in Washington, he's taken under the wing of Senator Joseph Paine, a childhood hero of Smith's. Paine, along with other members of the state's political machine, plans to use Smith's inexperience to their advantage, to pass a bill containing a graft scheme for a construction project they plan to profit from. Unwittingly at first, Smith plays into their hands, driven by his ambition to build a national boys' camp.

As Smith becomes more aware of the rampant corruption and the true intentions behind the proposed bill, he's urged to play along. However, his resolve strengthens when his secretary, Clarissa Saunders, who is cynical of all things Washington, provides him with insight into the political machinations at play. She comes to admire his naivety and determination and guides him in the right direction. Smith embarks on a journey to do what he believes is right, standing up for the principles of democracy and integrity against a backdrop of deceit and manipulation. His youthful optimism and unyielding spirit begin to win over those who had initially dismissed and tried to manipulate him.