France, 1789. In the home of the countess of Coigny, Carlo Gérard and the other servants are preparing for the evening festivities. When left by himself, Gérard airs his antipathy towards the aristocracy and predicts the impending doom of the the ruling class. The countess, accompanied by her daughter, Maddalena, enters the ballroom to check that everything is ready to receive the guests. Gérard ponders the girl's beauty. The guests arrive. They include Fléville, a novelist, who enters the room together with his friend, the poet Andrea Chénier. One of the invitees, a priest, brings disturbing news straight from Paris. A short dramatic work follows: a performance of a pastoral play by Fléville. The countess also asks Chénier to recite a poem, but he refuses. Maddalena begins to taunt the poet, provoking him until he finally relents. He delivers a verse that starts with a romantic theme and then quickly becomes political, ending as a parable against tyranny. The aristocrats are scandalized, but the poet's words have had a great impact on Maddalena. In order to relieve the tension, the guests move on to the dancing, which is interrupted by a crowd of famished people. Gérard has let them into the house, but then chases them back out at the command of the shocked countess, whose service he leaves as he departs together with the unwanted visitors. The guests attempt to lift their spirits with another dance.
Paris, 1794. The city is under Robespierre's reign of terror. Maddalena and her servant, Bersi, have come to the capital, where Bersi has become a courtesan in order to support both of them. The girl chats with an informer as they both watch Chénier, who is sitting nearby, and his friend Roucher. Roucher advises Chénier to flee Paris, but the poet's heart compels him to stay: for some time he has been regularly receiving letters from a mysterious, unknown girl. Roucher convinces him that mysterious girl is most certainly a woman of loose morals, and so the disappointed Chénier decides to abscond with a fake passport. Gérard, now one of the leaders of the Revolution, instructs the informer to find Maddalena for him, which the spy promises to do. Bersi approaches Chénier and delivers a message from her mistress: she asks the poet to wait for her. Roucher and the spy observe from the distance. Maddalena appears dressed as a maidservant, and when she quotes to Chénier from the poem she had heard him deliver at Madame de Coigny's ball five years earlier, the poet recognises the girl. Maddalena, who has been living in great danger, finds refuge in Chénier's arms, and the two profess their love for each other. The informer tells Gerard that he has found the lady, and he and Gérard arrest the couple before they can commence their escape. The poet asks his friend to look after Maddalena; he himself starts to fight a duel with the revolutionary leader. Gérard is wounded and, recognising the poet, encourages him to flee, as his name is included on the list maintained by Fouquier-Tinville, the public prosecutor of the revolutionary tribunal. Andrea Chénier escapes, but Gérard tells the crowd gathering around him that he has been wounded by an unknown assailant. The mob blames the deed on the Girondistes.
In the waiting room of the Revolutionary Tribunal, the somewhat inebriated Mathieu Populus is speaking to the people. The homeland is in danger: France is under attack by all of Europe. Gerard, who has recovered from his wounds, appears and continues Mathieu's speech. The people are moved by it and toss their last valuables into the collection box. An old and blind woman named Madelon even offers up her grandson, the last living male member of her family, for the defence of the country. The lad is taken up into the register and the crowd is soon out on the street singing La Carmagnole, the revolutionary anthem. The indefatigable informer brings more news: Chénier has been captured. Although Maddalena has disappeared, the spy reassures Gerard that if the case against the poet is strong enough, he can be condemned to death, and in that case there will be no need to look for Maddalena, since she will come on her own to beg for her beloved's life. Gérard writes out the charges against Chénier and, although he does not intend to behave dishonourably, his passion for Maddalena finally gets the better of him. Just as the spy predicted, Maddalena arrives. Gérard's crazed confession of love pushes the girl, who has already suffered a great deal, into a state of despair. Everyone around her has died, her mother has been killed, Bersi has taken up a life of sin for her sake and Chénier is languishing in prison. In exchange for her beloved's life, she is willing to give herself to Gerard. Gerard undergoes a change of heart and promises Maddalena that he will save Chénier even at the cost of his own life. The people stream into the chamber, curious to watch the hearings and executions. The jury and the president appear, along with Fouquier-Tinville, the public prosecutor. Finally, the accused are led in. Chénier is accused of treason, as well as of writing seditious poems against the revolution. The poet proclaims his innocence, but to no avail: the tribunal does not believe him. Gérard appears to testify in his favour, admitting that he was the one who wrote the charges and that he is now publicly withdrawing the allegations. Fouquier-Tinville remains implacable, and there are murmurs of bribery among the people mention bribery. The jury's verdict: death.
At the Saint Lazare prison. Andrea Chénier completes his final poem and shows it to his friend, Roucher. It is getting late, and the guard sends his friend away. Gérard escorts Maddalena to the prison. With gold and money, she pays off the guard so that when the executions are being carried out and the name of Idio Legray is called, she can take the condemned prisoner's place and thus die together with her beloved. The lovers meet in the cell and prepare for death together. It is growing light, and the hour of judgement is at hand.