Scene 1: The palace of the Count of Toulouse
Late at night Hélène is with her lover, Gaston. His family and hers have long been in conflict, but on the following day and prior to Gaston's departure for the First Crusade, it has been agreed that there will be a solemn reconciliation between the two families. (Duet: Adieu, mon bien-aimé / "Farewell, my beloved!"). After he leaves, Hélène and her companion, Isaure, pray for Gaston's safety.
Scene 2: The following morning outside the chapel
The Count proclaims the reconciliation and gives Gaston Hélène's hand in marriage. However, standing to one side, Roger, the Count's brother is quietly furious, since he is in love with Hélène. He leaves, after which the Papal Legate announces that the Pope has appointed Gaston to lead the Crusade, Gaston swears to follow him and he is awarded the Count's white cloak as a symbol of his loyalty. They enter the chapel. Returning, Roger proclaims his hatred of Gaston (Aria: Oh dans l'ombre, dans la mystère / "Oh! In darkness and mystery remain hidden, guilty passion") and approaches a soldier with whom he has plotted to kill his rival. He instructs the soldier to kill the man who will not be wearing the white cloak. (Aria: Ah! Viens, démon, esprit du mal / "Ah, come, demon, spirit of evil").
From inside the chapel the sound of uproar is heard. The soldier-murderer rushes out pursued by others while Roger gloats in his triumph. But it is Gaston who emerges, announcing that the Count has been struck down. The captured murderer is brought in before Roger; quietly Roger arranges for him to point to Gaston as the one who instigated the murder. Although protesting his innocence, Gaston is cursed by all and ordered into exile by the Papal Legate.
Scene 1: A cave near Ramla in Palestine
Remorseful, Roger has been wandering for years in the desert and he cries out for forgiveness. (Aria: Ô jour fatal, ô crime / "O dreadful day, o my crime!"). Unexpectedly, Raymond, Gaston's squire, appears in a state of exhaustion and he begs Roger, whom he believes to be a holy man, for help, telling him that others of his Crusader group are lost. Roger hurries off to help them. Hélène and Isaure come down the pathway seeking the hermit who they think can tell them of Gaston's fate. They are surprised to find Raymond, who tells them that Gaston is still alive but held captive in Ramla. While expressing her joy, Hélène and Isaure are led towards the town by Raymond (Aria: Quell'ivresse, bonheur suprême / "What rapture! Supreme happiness! God has protected the man I love...").
A group of distraught pilgrims climbs down from the hills around the cave. They are met by a band of newly-arrived Crusaders led by the Count, who praises God for saving him from the assassin's dagger, and the Papal Legate. Roger appears requesting that he may be allowed to join them in their battle and the three proclaim their anticipated victory (Trio and chorus: Le Seigneur nous promet la victoire! O bonheur! / "The Lord promises us victory! Oh joy!").
Scene 2: The palace of the Emir of Ramla
Gaston is admitted and expresses his desire to be close to Héléne again. He begins to plan his escape (Aria: Je veux encore entendre.. / "I want to hear your voice again") when the Emir arrives and advises him that escape will result in his death. At that moment, Hélène, having been captured in the city, is brought in. The couple pretends not to know one another, but the Emir is suspicious. However, they are left alone and are joyous in their reunion, until Gaston attempts to dissuade Hélène from associating with him in his dishonor (Aria: Dans la honte et l'épouvante / "You cannot share in the horror and shame of my wandering life!"). She remains firmly resolved to remain with him. From a window, they see the approaching Crusaders and, in the chaos, determine to escape but are thwarted by the arrival of the Emir's soldiers.
Scene 1: The harem gardens
Hélène is surrounded by the ladies of the harem who express some sympathy with her plight. But, when the Emir enters and is told that the Christians are close to attacking the city, he orders that if the invaders are successful, they shall throw Hélène's head to the Count. In despair, she considers the uselessness of her life (Aria: Que m'importe la vie / "What does life matter to me in my extreme unhappiness" and Mes plaintes mes plaintes sont vaines / "My laments are in vain").
Gaston has escaped and rushes in to find Hélène, but their joy is short-lived as the Crusaders, led by the Count, burst into the room and demand the Gaston's death, still believing that he was responsible for the attempt on the Count's life. Defiantly, Hélène challenges the Crusaders (Aria: Non...non votre rage / "No...no, your anger, your unworthy outrage") as well as her father ("The shame and crime are yours!"). The Count drags her away.
Scene 2: A scaffold in a public square in Ramla
Gaston is brought in and the Legate tells him that he has been condemned by the Pope and, following his public disgrace that day, he will be executed the following day. Gaston pleads for his honor to remain intact (Aria: O mes amis, mes frères d'armes / "O my friends, my brothers-in-arms"), but the smashing of his helmet, shield, and sword take place.
Scene 1: The edge of the Crusaders' camp
The hermit Roger is alone near the camp. A procession of Crusaders and women arrive, Hélène amongst them. The procession continues on, although Hélène hangs back as the Legate approaches Roger and asks him to give some comfort to the condemned man who is then brought out. Gaston is brought out, but Roger refuses to give him his blessing. Instead, he hands his sword to Gaston urging him to place his hands on its hilt where it forms a cross and then to go off and fight for the Lord.
Scene 2: The Count's tent
Hélène and Isaure wait for news of the outcome of the battle for Jerusalem. They hear shouts of victory from outside and the Count, the Legate, and Crusaders enter followed by Gaston with the visor of his helmet closed. Praised for his bravery and asked to reveal his identity, Gaston announces that he is now prepared to be executed. Just then, the mortally wounded Roger is brought in and reveals himself as the Count's brother. He begs for mercy for Gaston and confesses to being the one who planned the murder which almost resulted in his brother's death. All rejoice at the restoration of Gaston's honor and position, as Roger takes one final look at Jerusalem and he dies.