Act I
Violetta Valery, an unusually beautiful and successful courtesan, is hosting a party. Among the guests are Baron Douphol, her protector, Dr. Grenvil, her physician, and a young man from Provence, Alfredo, who is infatuated with her, though they have not yet met. When he is introduced, the two entertain the guests with a drinking song ("Libiamo ne' lieti calici"). As the guests leave the salon for the ballroom, Violetta is stopped by a fit of coughing, the result of consumption, which she has been fighting for more than a year. She dismisses her doctor and bids the guests leave her for a moment, after which she will join them. Alfredo remains behind. When Violetta has recovered enough to join her guests, he comes forward and announces his tender feelings toward her. She tells him that he may return to visit her the next day, and he departs with the rest of the guests. Left alone, Violetta muses on the possibility of experiencing real love, something, as a courtesan, she had not supposed to be in her future ("Ah fors'è lui che l'anima"), but immediately dismisses such a possibility as folly ("Sempre libera").

Act II, Scene 1
Violetta has rented a country house outside of Paris and is living with Alfredo. Alfredo expresses the joy he has found with this beautiful woman ("De' miei bollenti spiriti"). When he discovers that Violetta is financing their idyllic country life by selling her jewels and property, he leaves for Paris to raise money. Violetta enters and is told that Alfredo has gone to Paris for the day. A servant delivers a letter from her friend Flora, and before he leaves she tells him to expect a businessman who should be given immediate entry. The letter is an invitation to a fete that evening. Violetta is surprised her country retreat has been found out and laughs at the idea of returning, even for an evening, to her former life.

In moments, a gentleman is admitted, but it is not the expected businessman; it is Giorgio Germont, Alfredo's father. He has come to tell her to abandon her relationship with Alfredo. She had long expected this confrontation, but she did not expect to hear of Alfredo's young sister. The girl's proposed marriage is soon to be called off due to the scandal created by Alfredo's open relationship with a courtesan. Realizing that not only would Alfredo's future prospects be ruined by their affair, but that his entire family would suffer, she agrees to end the romance. When Germont leaves, she writes a note to Alfredo telling him that she is leaving him.

Alfredo enters and she hides the note, greeting him with tears. She tries to excuse her excited state, begging him to love her as she loves him. She then runs into the garden. Alfredo waits for his father who is due to arrive that day. His father comes in with the note from Violetta, which he delivers to his son. Alfredo is in despair and his father asks him to return to Provence (Di Provenza il mar"), but the boy flies into a rage. Finding the invitation from Flora, he plans to go to the party to exact revenge.

Act II, Scene 2
A party is in progress and a brief entertainment by dancers and guests posing as Gypsies and toreadors is performed. Alfredo enters without Violetta, who comes in shortly, accompanied by Baron Douphol. Alfredo and the Baron enter into a card game and Alfredo has a lucky streak, winning large sums with every hand. The guests go into an adjoining room for dinner and the stage is left empty. Violetta enters. She has asked Alfredo to join her, which he does momentarily. She asks him to leave the party, as he is insulting the Baron. He asks her if she loves the Baron. When she responds that she does, he calls in the other guests and throws his winnings in her face, repaying her for their time together. She faints and the Baron challenges Alfredo to a duel.

Act III
Near death, Violetta is in bed and her servant sits nearby. Doctor Grenvil enters and encourages Violetta with promises of recovery, but he whispers to the servant that there are only hours remaining. Alone, Violetta reads again, in a letter she has recently received from Germont, that Alfredo has been told the whole truth and father and son will soon arrive at her side. She knows it is too late ("Addio del passato"). Outside, a chorus of carnival revellers is heard and Annina is sent out to distribute alms. The servant returns very excited: Alfredo has arrived. He runs in and the two are rapturously reunited. They plan to return to that same country house and remake their lives, but this is soon clearly impossible. Germont and Dr. Grenvil join them. Violetta, at the moment of her death, feels her strength returning and stands thinking she will recover, but it is a trick of her illness. She falls and dies in Alfredo's arms.