ACT III - Scene 1. In Italy, near Velletri during the War of the Austrian Succession
As soldiers carouse in the background, Don Alvaro reveals in a soliloquy that he is the son of a Spaniard who had married the daughter of the last of the Incas and tried to free Peru from Spanish rule. His parents had been defeated, put in prison, where Alvaro was born, and executed, while he was brought up in the wilderness. Unaware that Leonora is still alive, he prays to her to look down on him from heaven. Disturbed by sounds of quarrelling and a cry for help, he rescues Don Carlo from the consequences of a quarrel over a game of cards. Excusing himself for being in such low company, on the grounds that he is but recently arrived, Carlo identifies himself as Don Felice de Bornos, aide-de-camp to the general, and Alvaro gives in reply the name he has assumed, Don Federico, Herreros, captain of grenadiers and, as Carlo exclaims in delight, the pride of the army. The two swear eternal friendship and go into battle together.
Alvaro is wounded and Carlo exhorts the surgeon to save him, promising Alvaro the order of Calatrava for his bravery. Feeling death near, Alvaro begs Carlo to burn unopened a packet of documents he will find among his possessions, and Carlo swears to obey; but while the surgeon is operating, doubts occur, spurred by Alvaro's horrified reaction to the name of Calatrava. He is tempted to open the packet, but his sense of honor restrains him. But near the packet he finds a portrait of Leonora and his suspicions are confirmed, and he receives with joy the news that Alvaro will live - so that he can kill him.
Scene 2. The camp near Velletri
The sun rises on bustling camp activity. Among those present is Preziosilla, telling fortunes, Trabuco, trafficking with the soldiers, and Melitone, reproving everyone for pagan goings-on on Sunday. When the soldiers turn on him, Preziosilla averts their wrath by embarking on a rousing rataplan.
ACT IV - Scene 1. The courtyard of the monastery of Hornachuelos five years later
Brother Melitone is dispensing food to the poor, complaining as he does so, so that they compare him unfavourably with the charitable Father Raffaele. When they have gone he discusses Father Raffaele with the Father Superior, explaining that he seems more like the devil than a member of a monastic order. Don Carlo knocks at the gate asking for Father Raffaele (Alvaro) and when they are alone confronts him, wishing to resume the interrupted duel - he has even brought two swords. But Alvaro has withdrawn from the world and tries to avoid the conflict. Rising to Carlo's taunt on his ancestry, he gains control of himself, but a blow cannot be overlooked and they run off to fight to the death.
Scene 2. A mountain gorge near a cave in the vicinity of the monastery
Leonora, dressed as a hermit, appears from the cave, praying for peace of mind: she has been unable to forget Don Alvaro. The sound of fighting disturbs her and she calls an imprecation on the heads of those disturbing her holy refuge. But the voice of the dying Carlo is heard calling for confession and Alvaro comes to beg the hermit to give him the last rites. They recognise one another and Alvaro tells her her brother lies dying. She goes to him, but he stabs her as he dies.
As she reappears, supported by the Padre Guardiano, Alvaro curses his fate and heaven, but is reproved by the Padre Guardiano, and Leonora assures him that heaven will pardon him. As she dies, Alvaro laments that he, the guilty one, lives on.
