The country estate of the Coigny family, the winter-garden of the chateau, 1789. A major-domo is supervising the preparations for a party. Carlo Gérard, one of the lackeys, breaks into a harangue against the aristocrats, an invective made more bitter by the sight of his old father, a gardener, struggling with a heavy weight. He hates the house and all in it - except, as he reveals when she appears with her mother, Maddalena, the daughter of the house, whose beauty charms even his embittered soul. Maddalena is still not dressed, but, as she tells Bersi, her mulatto maid, she hates the restrictive fashions and hairstyles of the day and intends to wear a simple white dress and a rose in her hair. The guests arrive and the novelist Fléville introduces the young poet Chénier. Discussion of affairs in Paris indicates that the country is in turmoil, but the guests determine to enjoy the evening. When Chénier evades an invitation from the countess to recite some of his poetry, Maddalena makes a bet with her friends that she can make him say something poetic; but he declines her invitation also, saying that the muse of poetry is shy like love.  Maddalena laughs, because he fell into her trap and used the word "love"; and Chénier, stung, promises to make clear to her the real meaning of love, and recites a poem in which he describes the poverty of the people and the indifference of the aristocracy to their sufferings, ending with an attack on Maddalena, telling her that she too, young and beautiful as she is, does not know the real meaning of love. She is abashed, the guests are indignant, and only Gérard, listening in the distance, is moved. A dance is about to begin, but a crowd of wretched people approaches the chateau and Gérard admits them, declaring his solidarity with them and tearing off his livery and compelling his father to leave with him. The peasants leave and there is an attempt to resume the festivities. 

Paris, June 1794. Mingling with the crowd and disguised in the extravagant costume of a merveilleuse, Bersi suspects that she is being watched by a spy dressed in the equally fantastic garb of an incroyable. She accosts him and explains defensively that she is, like him, a true child of the revolution. The spy suspects a connection between the beautiful blonde he has seen with her and Chénier, whom he has been watching. Chénier's friend Roucher brings him a passport, but Chénier, although in danger, refuses to leave. He believes that he is on the threshold of an adventure, as he has been receiving mysterious letters from a woman who signs herself Hope. But Roucher, basing his opinion on the scented notepaper, convinces him that it can only be a woman of the street and he agrees to take the passport and escape. Acclaimed by the crowd, Robespierre and other Representatives appear, among them Gérard, who is called aside by the spy who tells him that he has found the woman Gérard is searching for. Despite being beset by the spy, Bersi manages to tell Chénier that a woman who is in great danger, whose name is Hope, wants to speak to him and he agrees, despite Roucher's warning that it is a trap. Watched by the spy, Maddalena, disguised as a servant, arrives. As Chénier recognizes her, the spy goes off to tell Gérard. Maddalena explains that she had hesitated to approach Chénier when he was powerful and she was in danger, but had often written to him, as to a brother; but now she feels she is in danger and asks for his help. Realizing that she is to be the love of his life, Chénier agrees to help her. They declare their mutual love and their intention of staying together, but Gérard, warned by the spy, appears and tries to claim Maddalena. She escapes with Roucher, who has been keeping guard, while Chénier and Gérard fight. Gérard is wounded, but recognizing his opponent he warns him to escape and protect Maddalena; and when the crowd rushes to his aid, tells them that he did not know his assailant. 

At the Revolutionary Tribunal, waiting for a trial to begin, the sans-culotte Mathieu calls on the crowd to make donations to save their country from foreigners, abusing them when they do not respond; but when Gérard, recovered from his wound, appeals to them, they offer their jewelry for the cause. An old blind woman, Madelon, brings her young grandson, whose father was recently killed in battle, and offers him to the Republic. The spy reports to Gérard that Chénier has been arrested, but Maddalena has disappeared; but he is convinced that the news of his arrest will bring her out of hiding. Gérard prepares the indictment against Chénier, torn between his belief that Chénier is no traitor (although he is on the public prosecutor's list) and his passion for Maddalena, reflecting on how this passion has adulterated the purity of his revolutionary fervor and his concern for the downtrodden. Maddalena comes to plead for Chénier and Gérard admits to having had her followed and that, far from hating her as she thinks, he has loved her since childhood. Maddalena understands him and offers herself in exchange for Chénier's life, telling him of how her mother had died saving her from the mob, how Bersi had sold herself to provide for her and how love had come to her in her wretchedness. The list of those to be tried is brought to Gérard, and Chénier's name is on it; but Gérard, moved by Maddalena's appeal, promises to try and save him. The trial begins and after some hasty condemnations, it is Chénier's turn. He defends himself against the accusation, saying that as a soldier he had confronted death proudly and as a poet has used his pen against hypocrisy. Gérard interrupts the proceedings to claim that the indictment he wrote is a lie, and that Chénier is a true son of the revolution, but he is disregarded and the death sentence is pronounced. 

At the prison of St Lazare, Roucher is taking leave of Chénier, who reads him his last poem, which he has just written and in which he compares his last moments to the end of a lovely May day. Gérard brings Maddalena, who bribes the jailer to let her take the place of a young woman, Idia Legray, who had been condemned that day. Gérard rushes away to try to see Robespierre and save her life and Chénier's. Chénier and Maddalena are blissful in their reunion and confront the prospect of death bravely, answering their names proudly when they are called to the tumbril.