Ferrando and his men, soldiers of Count di Luna, fortify themselves after a vicious battle in Spanish Aragon. To keep the men alert and on guard, Ferrando reminds the men of the reason they fight: the story of the two sons of the former Count (father of the current Count). Long ago an old traveling peasant woman was accused of bewitching the Count’s younger son, still an infant, who had turned sick. The witch was burned. In revenge, the woman’s daughter abducted the sickly baby and beside the witch’s bones were found the charred remains of a child. The old Count never believed his son had been killed and, on his deathbed, made his surviving son swear an oath that he would never give up the search for his younger brother. Ferrando, who witnessed these events, swears that he would recognize the witch’s daughter to this day.
Elsewhere in the palace, Leonora, a lady-in-waiting to the princess, tells her friend Inez about a mysterious troubadour who serenades her each night.
Count di Luna, who is infatuated with Leonora, watches her lustfully in secret. As the troubadour begins serenading Leonora, she rushes towards his voice. But in the darkness, she mistakes the Count for her love. The troubadour corrects her mistake by revealing himself: Manrico, champion of the rebel army. The Count challenges him to a duel.
As dawn rises on a rebel encampment, Manrico’s rebels sing the Anvil Chorus, working endlessly to create more weapons for the continuing war. Azucena, a troubled old woman, sings a haunting story about an innocent woman burning at the stake. Manrico is left alone with Azucena, his mother, who found him on a battlefield left for dead. He asks her why she always sings that song. Azucena explains that her mother was burned to death by the Count’s men. As she watched her die, Azucena heard her mother scream “Avenge me!” Driven by the plea, Azucena abducted the Count’s infant brother, with the intention of burning him too. She took the baby as well as her own child to the pyre. But the baby began crying, and she found she could not go through with the crime. Her mother’s screams and the baby’s wailing overwhelmed her senses, and in her frenzy she pushed a child into the fire. She recovered her sanity and realized, in horror, that she has killed her own son.
Hearing her delirious story, Manrico is hurt. Who is he if not her son? Azucena insists that he is her son. She begs Manrico to avenge his grandmother by killing the Count. She asks why he failed to do so in the duel. Manrico explains that he was ready to strike the killing blow when a mysterious power stayed his hand.
A messenger brings Manrico news that his beloved Leonora, believing him dead, is going to enter a convent. Despite Azucena’s pleading that he stay, he rushes off to find Leonora.
Count di Luna also hears about Leonora’s plans and arrives at the convent with a group of men to abduct her. Manrico and his men arrive and he escapes with Leonora amidst the melee.
Di Luna’s men prepare to attack the fortress where Manrico has taken Leonora. They find Azucena, who is wandering nearby desperately searching for her son. She is brought to the Count and Ferrando recognizes her as the witch who killed the Count’s infant brother. In desperation, Azucena screams for help, yelling her son’s name. The Count realizes she is Manrico’s mother. Di Luna has her tortured, setting in motion a plan that he hopes will trap his rival and avenge his dead brother.
Leonora and Manrico, cowering in the chapel of their besieged fortress, pledge themselves to one another when they learn that the Count has captured Azucena. Manrico gathers his remaining forces and rushes off to mount a desperate rescue.
But he’s captured and awaits execution in di Luna’s prison along with Azucena. Leonora offers herself to the Count in exchange for Manrico’s freedom. Di Luna agrees, so long as she never see Manrico again. But once his back is turned, Leonora swallows a dose of deadly poison.
In their prison cell, Manrico tries to comfort Azucena, who keeps seeing the vision of her mother burning at the stake. Leonora comes to release Manrico, who guesses at the bargain she has struck with di Luna. Outraged by what he perceives as her infidelity, he refuses to flee, despite her entreaties. Only when she succumbs to the poison does he understand what she has done. Di Luna enters the prison as Leonora dies in Manrico’s arms, and the Count forces Azucena to watch Manrico’s execution. Azucena tells the Count that he has murdered his brother. Her mother is finally avenged.