Scene 1: A square outside the church of Sant' Ambrogio in Milan
The two sons of Lord Folco, Pagano and Arvino, are reconciled, having previously feuded over which man would win the hand of Viclinda. Pagano, who once threatened the life of his brother, has returned from exile. A throng gathers in front of the church of Sant'Ambrogio to celebrate. Viclinda, now Arvino's wife, and their daughter Giselda are on hand to witness the reconciliation. A crusade to the Holy Land is announced and Arvino is to lead it. Pagano secretly vents his enduring frustration to Pirro, Arvino's squire: he still desires Viclinda (Sciagurata! hai tu creduto / "Wretched woman! Did you believe that I could forget you..."). As nuns sing in the background, Pirro and a gang of cut-throats agree to help Pagano take Viclinda for himself.
Scene 2: The Folco palace
Viclinda and Giselda are concerned about Pagano and his supposed reformation. Arvino asks them to watch his elderly father, Lord Folco, who is spending the night in Arvino's chambers. Giselda prays (Aria: Salve Maria / "Hail Mary!"). Pirro and Pagano and their assassins storm the palace. Pagano draws his sword and enters Arvino's chambers. He emerges with a bloody sword and with Viclinda in his custody. Arvino suddenly appears and Pagano is shocked to learn that in the darkness he has killed his father, not his brother (Orror! / "Horror! Dreadful monster of Hell..."). A throng calls for Pagano's death, but Giselda protests against more bloodshed. So Pagano is once again sent into exile.
Scene 1: Acciano's palace in Antioch
Acciano and representatives from surrounding territories plot their continued resistance to the marauding crusaders. They have captured Giselda, who is now held captive within Acciano's harem. Sofia, Acciano's head wife and a secret Christian, enters with her son Oronte. Oronte has fallen in love with the captive Giselda (Aria: La mia letizia infondere / "Would that I could instill my gladness into her dear heart"). As Oronte sings of his love, Sofia sees Giselda as a means of converting her son to Christianity (Come poteva un angelo / "How could Heaven create an angel so pure").
Scene 2: A cave in the desert outside Antioch
A hermit waits for the arrival of the crusaders. A man appears at the cave and asks the hermit how he may receive forgiveness for his past sins. The man is Pirro, who has become a confidante of Acciano and now controls the gates of Antioch. The hermit counsels Pirro that he will achieve forgiveness if he opens the gates to the approaching crusaders. Thereafter, the crusaders, led by Arvino, appear at the cave. The hermit learns that Arvino's daughter has been captured by Acciano. The hermit assures them that they will succeed in taking Antioch.
Scene 3: Acciano's harem
The members of the harem sing of Giselda's luck in attracting the attentions of Oronte. As Giselda prays (Aria: Oh madre, dal cielo / "O mother, from heaven hear my lament") sudden shouts warn that the crusaders have invaded Antioch. Sofia rushes in to say that both Acciano and Oronte have been killed. Arvino enters with the hermit. Sofia identifies Arvino as the murderer of her husband and son. Giselda is horrified and recoils at her father's attempted embrace. She declares that this crusade was not the will of God. Arvino draws his sword and threatens to kill her for her blasphemy, but he is stopped by the hermit and Sofia. Arvino declares that his daughter has gone mad.
Scene 1: The valley of Jehoshaphat; Jerusalem is in the distance
The crusaders, joined by Christian pilgrims, sing of the beauty of Jerusalem and the Holy Land. Giselda has wandered away from her father's camp. Suddenly, Oronte appears! He was merely wounded, not killed, by Arvino's sword. Giselda and Oronte decide to flee together (Duet: Oh belle, a questa misera / "O Lombard tents, fair to this unhappy girl, farewell!").
Scene 2: Arvino's tent
Arvino rages against his daughter's betrayal. Soldiers arrive to tell him that Pagano has been seen in camp and they call for his capture and death. Arvino agrees.
Scene 3: A grotto near the River Jordan
After a violin prelude, Giselda and Oronte appear. Oronte has been wounded and Giselda bitterly laments God's cruelty. The hermit appears. He tells Giselda and Oronte that their love is sinful but may be purified by Oronte's conversion and baptism. The hermit completes the baptism, and Giselda laments as Oronte dies from his wounds, promising to see her in heaven (Trio: Qual volutta trascorrere / "What wondrous pleasure I feel").
Scene 1: A cave, near Jersulam
As Giselda dreams, Oronte appears to her in a vision and tells her that God has granted his prayer: the crusaders will find strength in the waters of the fountain of Siloam (Aria: In cielo benedetto / "Through you, Gisleda, I am blessed in heaven"). Giselda wakes and sings of her miraculous vision (Aria: Qual prodigio . . . Non fu sogno! / "A miracle!... It was not a dream").
Scene 2: The Lombards' tents
The crusaders and pilgrims are despairing that God has abandoned them in the desert (O signore, dal tetto natio / "O Lord, Thou dids't call us"). Giselda rushes in, announcing the discovery of a spring of water. As all rejoice, Arvino assures his crusaders of his confidence that they will now take Jerusalem.
Scene 3: Arvino's tent
Dying from wounds, the hermit is brought in by Giselda and Arvino. The hermit reveals that he is really Pagano. In his dying moments, he confesses to Arvino his penitence for their father's murder and begs forgiveness. Arvino embraces his brother, and Pagano asks for a final view of the Holy City. As Jerusalem appears in the distance, Pagano dies, and the crusaders praise heaven (Te lodiamo, gran Dio di vittoria / "We praise Thee, great God of Victory").