Scene 1: Milan, not far from the city walls
Arrigo, a young soldier who is believed by all to be dead, is part of the forces of the Lombard League which has assembled to set out on a campaign against Frederick Barbarossa, the German Emperor. He recounts how his mother nursed his wounds (La pia materna mano / "A mother's kindly hand") after he was left for dead and, having recovered and before leaving again, he wants to see Lida, his sweetheart. A colleague, Rolando, who leads soldiers from Milan, arrives and, amazed, recognizes Arrigo (Ah m'abbraccia d'esultanza / "Ah! Come to my arms..."). The gathered troops all swear to defend Milan against the tyrants.
Scene 2: Beside the ramparts of the city
Lida, Rolando's wife, is downcast at the prospect of further war. A German prisoner, Marcovaldo, who has been given some degree of freedom by Rolando, declares his love for Lida, but she is disgusted. Then she is appalled to learn that her husband is returning home with none other than Arrigo. (A frenarti o cor nel petto / "My heart, no longer have I the power..."). Upon his arrival, Arrigo is clearly upset to see Lida. Rolando is called away and, left alone with Arrigo, she tries to explain that it was her father who encouraged her to marry Rolando after all believed that Arrigo had been killed in battle. Declaring her a "faithless one" Arrigo hurries away wishing only to die in battle.
The town hall of Como The city fathers have gathered to hear the news that Milan has been forced to come to terms with the invaders. Then Arrigo and Rolando arrive to announce that a new army has invaded from the north and seek Como's help, pointing out that the city lies between Milan and the invaders. They hope that Como will intervene to help the Italian cause. Suddenly, Barbarossa himself appears proclaiming himself as "I am Italy's great destiny", his men having surrounded the city and now further threaten Milan. He demands that Arrigo and Rolando return to Milan and seek its submission.
Scene 1: The basilica of Sant' Ambrogio
Arrigo joins the Knights of Death, a group which pledges itself to fight to the death rather than suffer defeat or imprisonment. All unite to swear an oath to support the cause of Italy (Giuriam d'Italia por fine ai danni / "We swear to put an end to Italy's wrongs").
Scene 2: Rolando's castle
Lida has heard of Arrigo's attachment to the Knights of Death and desperately tries to contact him via a note conveyed by her maid, Imelda. As Imelda is about to leave, Rolando sudenly arrives to say farewell to Lida and to his son who he demands to be brought to him. Imelda hides the note and then quickly leaves. He tells Lida to convey his feelings of love of his country and to bring the boy up to love the fatherland (Digli ch'e sangue italico / "Tell him he is of Italian blood").
Arrigo, who has been summoned by Rolando, who does not know of his allegiance to the Knights of Death. Thinking that Arrigo has been ordered to remain to guard Milan, Rolando begs him to take care of his wife and son in the event of his death: (Se al nuovo dý pugnando /"If when we fight on the morrow"). As Rolando leaves, Marcovaldo appears to warn him that his honor has been betrayed: the prisoner has intervened and taken the secret note which Lida wrote to Arrigo. He gives it to Rolando who angrily exclaims that he will obtain vengeance: (Mi scoppa il cor / "My heart is bursting").
Scene 3: A room in the tower
Arrigo is alone when Lida suddenly arrives to see Arrigo, not having had a response to her note. They declare a mutual love for each other but he tells her that he has not received any note, only to have Rolando attempt to burst into the room. Lida is hidden on the balcony, and Rolando confronts Arrigo telling him that he knows about his vow to the warriors of death. Opening wide the door to the balcony, Rolando discovers Lida. The men argue, but Lida declares that she is the guilty one. In a rage, Rolando storms out, locking the door, to lead his troops into battle, something which Arrigo can only watch from the balcony. Soon he plunges from it to join the soldiers below, leaving Lida in anguish.
A square in Milan
The assembled people sing for victory and Ismelda assures Lida that Arrigo was seen to escape from the battlefield. Lida prays for both men's safety: Ah se di Arrigo e Rolando / "Ah! If I recommend to Thy care the lives of Arrigo and Rolando". A hymn of victory begins and it is confirmed by the arriving officials, who enter the church declaring that Barbarossa was slain by Arrigo. Suddenly, a group of Soldiers of Death bring the mortally wounded Arrigo into the square. Rolando approaches him and Arrigo swears that Lida is innocent and proclaims with his last breath that "Italy is saved!".