When a Senate seat needs to be filled because of a death, Governor Hubert 'Happy' Hopper (Guy Kibee) appoints Jefferson Smith (James Stewart) to come to Washington. He is met by corrupt senior Senator Joseph Paine (Claude Rains) and political boss Jim Taylor (Edward Arnold). He was chosen because of his lack of experience so he won't get in the way of the self-serving plans of these men. Smith is idealistic about his role as Senator and the government and wants to do good but doesn't know the process. Smith meets a wise-cracking secretary Clarissa Saunders (Jean Arthur) who tries to educate him by teaching him how he can get a bill introduced. Smith decides to try to get a bill for a national boy's camp passed. He doesn't know that this bill will hinder the corrupt plans of his fellow politicians. Taylor first tries to squash Smith through bribery and intimidation, but then tries to ruin him by having Paine present unfounded charges against him to have him removed from his position. By this time, a grass-roots movement by boys across the country has spread and Smith knows he must fight for his bill for the boys. With Clarissa by his side, he presses on even if it costs him his reputation or his own life.