Upon the death of his state's junior senator, Jefferson Smith (James Stewart) is appointed to serve alongside Joseph Paine, a Washington D.C. legend who leads a corrupt political machine. Placing a lieutenant, Saunders (Jean Arthur), as Jeff's watchful secretary, Paine hopes to gain quick approval for an unnecessary -- but profitable -- dam by keeping Smith focused on a bill to build a boys' camp. After Saunders falls in love with her new boss, Jeff threatens to expose the scam, but is undermined when Paine claims that Smith owns the camp's land. But, when Jeff fights back with a heroic, and exhausting, filibuster, it sparks Paine's heartfelt confession, making Smith a national hero -- and Saunders' new husband.