The untimely death of Senator Foley presents problems for political boss Jim Taylor, who needed the senator's help to perpetrate a land swindle at Willet Creek. Taylor orders Governor Hubert Hopper, whom he controls, to appoint a yes man, but citizen committees want someone else. Hopper is also besieged by his sons, who ask him to appoint Jefferson Smith, the patriotic leader of the Boy Rangers. Confused, Hopper appoints Jeff, then convinces Taylor that naïve Jeff cannot learn enough about politics in time to affect the crooked bill. Jeff's appointment as junior senator is also supported by the senior senator, Joseph Paine, who is both Taylor's stooge and Jeff's idol. Jeff and Paine go to Washington, where Jeff, overwhelmed by his first sight of the Capitol dome, leaves the group and boards a tour bus. Five hours later, he reaches his office, where his cynical secretary, Clarissa Saunders, is waiting for him with her chum, newspaperman Diz Moore. They think Jeff's patriotic spirit is hokum, and Saunders engineers a disasterous press conference for Jeff. The next morning, Paine takes Jeff to be sworn in at the Senate, where one senator objects, alleging that the newspaper stories prove Jeff is unfit. Paine defends Jeff, and after he is sworn in, enraged Jeff goes on a rampage, slugging the reporters, who label him an "honorary stooge." The truth of it stings Jeff, and after seeking advice from Paine, who tells him to sponsor a bill proposing a national Boy Rangers camp, Jeff and Saunders stay up all night working on the bill, which Jeff presents in the Senate the next morning. Despite Jeff's nervousness, the senators like his ideas, except for Paine, who is horrified to discover that Jeff wants to use Taylor's Willet Creek site. Paine knows that Jeff must not be in the Senate the next day, when the Willet Creek bill is being discussed, and so he resolves to distract Jeff with his beautiful daughter Susan. Jeff is thrilled by Susan's attentions, but the next night, Saunders, drunk with Diz, becomes distraught over the way Jeff is being misled. She asks Diz to marry her, and they return to her office to collect her things. Jeff is there when they arrive, however, and she tells him about Paine, Taylor and the graft. As they leave, Diz realizes that Saunders is in no shape to get married, and he takes her home. Stunned by Saunders' revelations, Jeff rushes to Paine's house to confront him, but Paine tries to smooth-talk him. Later, when Taylor himself arrives, he tells Jeff that he runs Paine, and that if Jeff is smart, he will cooperate. The next day, Jeff attempts to speak against the crooked bill, but, not understanding rules of protocol, yields the floor to Paine, who denounces Jeff on charges of using the boys camp for personal gain. Some time later, at Jeff's hearing before the Committee on Privileges and Elections, Hopper, Paine and others present phony evidence that Jeff owns the land upon which he wants to build the camp. Jeff is so dumbfounded by Paine's lies that he cannot testify on his own behalf and decides to leave Washington. Later that night, Jeff goes to the Lincoln Memorial, where Saunders finds him and convinces him to attempt a filibuster. The next morning, after a night of coaching, Jeff reveals the truth about Taylor and Paine to the Senate, even as Paine continues trying to condemn him. Jeff intends to talk until his news reaches his home state, and the people rise up against the corruption, but Taylor organizes a massive newspaper campaign against Jeff. Many hours later, Saunders cheers up Jeff with a note telling him she loves him, and then calls his mother, telling her to enlist the Boy Rangers to spread the truth. The boys publish their tiny newspaper, but Taylor's gang steals the papers and injures some of the boys. Back at the Senate, Paine brings in 50,000 telegrams drummed up by Taylor, all of them urging Jeff to quit. Though discouraged, Jeff resolves to keep fighting, but after he gives one last speech to Paine, he collapses from exhaustion after the almost twenty-four hour filibuster. Paine finally breaks down, and after attempting suicide outside the senate chamber, confesses that everything Jeff has said is true. Everyone in the room cheers and Saunders jumps for joy.