In an unnamed western state, Governor Hubert "Happy" Hopper (Guy Kibbee) faces the daunting task of selecting a replacement for the recently deceased U.S. Senator Sam Foley. Under significant pressure from his corrupt political boss, Jim Taylor (Edward Arnold), who insists on choosing his own candidate, Hopper is also swayed by the popular demand for a reformer. His children suggest that he nominate Jefferson Smith (James Stewart), the dedicated head of the Boy Rangers. Caught between the two options, the governor resorts to flipping a coin and, when it finds an unlikely balance next to a newspaper article highlighting one of Smith's achievements, he makes the unexpected choice, believing Smith's clean reputation will win public approval while his naïveté will make him easily controllable.
Smith soon finds himself mentored by the seemingly honorable but actually corrupt Senator Joseph Paine (Claude Rains), a close friend of Smith’s late father. Additionally, he becomes infatuated with Paine's daughter, Susan (not portrayed by an actor listed in the provided information). However, the Washington press quickly dismisses Smith as a simpleton unfit for senatorial responsibilities. In an effort to keep him occupied, Paine suggests that Smith introduce a new bill.
Smith proposes legislation to secure federal funding for a national boys' camp in his home state, which he believes will be repaid by donations from youngsters across the nation. Initial support comes pouring in, but unbeknownst to him, the location for the camp is already entangled in a dubious public works project orchestrated by Taylor's political machine, with Paine’s tacit approval.
Paine, torn between his loyalty to Smith and the demands of Taylor, ultimately sides with the political machine. When they falsely accuse Smith of attempting to profit from the land deal, Smith is left reeling from this betrayal and flees in distress. However, Clarissa Saunders (Jean Arthur), Smith’s loyal chief of staff, believes in his integrity and persuades him to mount a filibuster on the Senate floor in an attempt to prove his innocence before the vote for his expulsion.
As Smith delivers impassioned speeches that seem to resonate with his constituents, the overwhelming opposition, fueled by the Taylor machine, works tirelessly to undermine him. Smith's pleas are systematically ignored by the media in his home state, thereby thwarting any support from the public. Even efforts by the Boy Rangers to promote his cause lead to brutal backlash from Taylor’s operatives.
Just when the odds appear insurmountable, attention begins to shift in Smith's favor as he approaches his physical limits during the filibuster. Paine makes a desperate move by presenting a stack of letters and telegrams from Smith's state demanding his ouster, leaving Smith utterly despondent. Yet in the midst of despair, he receives a glimmer of hope from a sympathetic smile from the President of the Senate, H.B. Warner (not portrayed by an actor listed in the provided information). Vowing to persevere until the truth prevails, Smith collapses from exhaustion.
Gripped by guilt over his actions, Paine attempts to take his own life but is thwarted. He subsequently bursts back into the Senate chamber, courageously confessing the nefarious plot against Smith and exonerating his innocence, turning the tide in this gripping tale of courage, corruption, and redemption.
