Based on the novel by George Gibbs and E. Laurence Dudley, Voltaire uses a play to expose the king's corrupt minister.
Count de Sarnac (Alan Mowbray) shows King Louis XV (Reginald Owen) a revolutionary pamphet he blames on Voltaire. Voltaire (George Arliss) writes the king to pardon Calas. Voltaire's doctor scolds him for drinking coffee. Sarnac tells the king his treasury is low, and he accuses Voltaire of giving Prussian king Frederick military information. The king orders Calas executed and gives his estate to Sarnac. Voltaire learns of the execution and sends for the proscribed daughter of Calas, taking her into his home. He writes to Madame de Pompadour (Doris Kenyon). The king forbids her to see Voltaire, but she visits him anyway. Nanette Calas (Margaret Lindsay) tells Pompadour how her father was tortured by Sarnac. Pompadour goes behind a curtain before Sarnac comes in. Sarnac demands Nanette. Voltaire denies she is there and asks that secret trials be abolished. Sarnac arrests everyone in the house, but Pompadour rips up the royal order. She tells Voltaire the king will see his play with Nanette. Voltaire sits down to write the play.
Voltaire's play is presented at the court in Versaille. In the play the king orders a merchant tortured, and his captain of the guard tortures him to death and gets his estate, as the man's daughter and the king's mistress were unable to stop it. In the audience Sarnac shows the king a pamphlet calling for the oppressed to rise. The king stops the play and arrests Voltaire. Pompadour gets him to let Voltaire go home first. Voltaire's servant got no evidence on Sarnac. The king has Voltaire's writings burned. Pompadour seeks shelter with Voltaire, who collapses and is dying. When he is told Frederick sent a gold present to Sarnac, he revives. Sarnac gives Voltaire the royal warrant for his arrest, but Voltaire accuses Sarnac of giving Frederick military information, betraying France. Voltaire tells the king of proof Sarnac is a traitor; Sarnac confesses, and Louis has him arrested. Voltaire burns Frederick's poetry, the supposed evidence. The king pleads with Pompadour, and Voltaire asks Louis to restore the estate of Nanette Calas and the throne of France to justice, warning of revolution. The final montage shows Voltaire writing about justice, tolerance, and liberty.
Although these dramatic events are fictitious, the wit and character of Voltaire are based on that versatile genius. With impending revolution the corruption of the court is portrayed. The story conveys Voltaire's zeal for humanitarian values and tolerance of dissent.