Paul Muni stars in the titular role; penniless and struggling with his writing as a young man in late 19th century Paris. His writings draw the ire of the French establishment, but a chance encounter with a street sex worker leads to Zola publishing his novel Nana, which gives Zola the success and cultural respect he has been seeking.
With each subsequent writing, Zola gains more attention from the public as well as the censors. Angering the French military and politicians with his novel Downfall, Zola’s riches lead him to a contented existence with his wife Alexandre (Gloria Holden).
Years later, French Army Captain Alfred Dreyfus (Joseph Schildkraut) is falsely convicted of treason and imprisoned. Amid public fervor for the conviction and despite evidence that would vindicate Dreyfus, the French generals decided to suppress the truth to avoid a public rebuke of their judgment. Alfred’s wife Lucie (Gale Sondergaard) pleads with a stagnant Zola to utilize his power and address this miscarriage of justice.
Zola publicly accuses a number of generals and officials in a scathing newspaper editorial. Charged with libel, Zola faces the French High Command and the President of the Republic to vindicate Dreyfus, while also trying to save himself and reinvigorate his passions.