1762. The lawyer Chalier visits Voltaire at his home in Ferney. Chalier tells the philosopher of a man unjustly executed because of his religion. He recalls how a cry drew a crowd to the home of Jean Calas in Toulouse. There they found the body of Calas's youngest son, Marc-Antoine, who had apparently been strangled. The town Capitoul, David De Beaudrigue, questioned the household and inspected the body.

Leaving the scene, the Capitoul heard accusations that Calas murdered his son to stop him converting to Catholicism. The Capitoul was shocked, previously unaware that the Calas family, with the exception of the older son Louis, were Protestant. He took the household into custody. Chalier recollects the examination of the Calas family and servants. De Beaudrigue zealously exploited the inconsistencies in Calas's statements. Ignoring his clerk Moynier's remonstrations that he was biased, the Capitoul was certain of Calas's guilt.

Voltaire remains unconvinced of Calas's innocence. Chalier recalls Calas admitting that he was guilty only of attempting to hide the fact that his son had committed suicide; to preserve his son's honour, the hanging body was cut down and the rope hidden. The Capitoul preferred to believe that Marc-Antoine was murdered by his father to stop him changing faiths. Louis visited his father's house with Chalier and located the rope and billet with which his brother hanged himself. Chalier knew that this evidence would not sway De Beaudrigue.

Having initially dismissed Chalier's story, Voltaire is haunted by thoughts of Calas's suffering and writes to a high minister, the Duc de Choisel, in Paris. The Duc is assured of Calas's guilt by Secretary of State Saint-Florentine. Refuting Saint-Florentine's reply to Voltaire, Chalier recalls how not 25 judges, but 13 heard the case, and only eight found Calas guilty. He explains how Laffiteau, the head of the fanatical White Penitents, declared that Marc-Antoine had been on the very point of converting, despite the insistence of others that he had consistently refused to do so. The zealously anti-Protestant Laffiteau urged his Catholic congregations to supply evidence of Calas's guilt to the Capitoul.

De Beaudrigue was overwhelmed by crowds citing gossip and hearsay, which he in turn interpreted to support his conviction. He refused Protestant witnesses and ignored all of Chalier's objections to his prejudiced procedure. Incensed by the Capitoul's lack of reason, Moynier abandoned his post. With the Capitoul's permission, Laffiteau's White Penitents led a Catholic funeral for the Protestant Marc-Antoine. They were interrupted by Chalier and Moynier but De Beaudrigue twisted the words of Louis Calas to suggest that Marc-Antoine's conversion was certain, and allowed the funeral to continue. Meanwhile, a court found Calas guilty, ordering that he be tortured to confession, broken on the wheel and executed.

Hearing Chalier's reading of the sentence, Voltaire becomes violently agitated and declares that with a public campaign he will find the truth. His provocation of public interest across the country agitates the Duc de Choisel. He orders the Secretary of State to write to placate Voltaire. Instead, Saint-Florentine has a dozen men despatched to silence him.

Voltaire pamphleteers with a passion, but the police suppress his literature. He is perturbed to hear of the strangers in the village and the rumour that an 'accident' awaits him. He decides to stay in the safety of his home. Meanwhile Saint-Florentine becomes aware that the French public, King and Queen are becoming sympathetic to the Calas family. Parliament calls for Calas's rehabilitation. Visiting Voltaire, Father Bourges recalls Calas's refusal to confess to any crime as he was tortured on the wrack. On the scaffold, his limbs were smashed before he was put to death.

1778. A town crier declares that Calas has been rehabilitated, his conviction quashed. De Beaudrigue is informed of the instruction. His realisation that he insisted on a Catholic funeral for a Protestant suicide causes him to fall from a window to his death. At his home, Voltaire rests, hoping that this resolution may act as a caution to all tyrants.