The Midianites’ encampment outside the walls of Memphis. Hebrews and Midianites together call upon God to help the people of the Hebrews to return to their own country: will the King of Egypt fulfil his promise of letting them go? Moses enters; he reproves them for their lack of faith and bids them hope: God is on their side, and he, Moses, will guide them. Having recovered from their temporary bewilderment, the people all feel more hopeful.
Moses is awaiting the return of his brother Eliézer, who has been sent to ask Pharaoh to bring to an end the exile of the Hebrews by letting them go. Soon Eliézer comes back, accompanied by his sister Marie ad her daughter Anaï, and tells them how he has pleaded their people’s cause before the King, how the High Priest, Osiride, has violently opposed them, and how Queen Sinaïde has resolutely supported them. Pharaoh, persuaded into a merciful frame of mind, has given them a sign of his good faith by releasing Marie, who had been condemned to death for having refused to worship the gods of Egypt. Marie descants upon the exemplary behaviour of her daughter, who has shown that her devotion to her God and to her mother mean more to her than her love for Prince Aménophis. Whilst Moses invites his people to rejoice, some supernatural phenomena take place: a rainbow, taken for a sign of God’s favour, appears in the sky; a meteor sets fire to a bush, which burns without being consumed by the fire. A mysterious Voice orders Moses to draw near and receive the Law; it announces that the Hebrews will be subjected to new tests, but will win through in the end. The fire burns itself out and the bush flowers anew: Moses approaches and from the branches draws forth the tables of the Law, which he shows to the people of the Hebrews. All kneel down, invoke God’s name and swear to obey His commandments. To seal this pact, all the firstborn sons of the Hebrews are consecrated to God. When the ceremony is over, Moses urges them all to get ready; soon they will set out on their journey.
Anaï is left alone onstage. Anaï is joined by Aménophis, who complains that she has deserted him in spite of all her promises to love him. Anaï protests that she loves him sincerely, but she feels even more strongly her faith in God, her sense of belonging to the Hebrews and of needing to obey her mother. Aménophis alternately begs and threatens her, defies Moses and all his family, but in vain: when Anaï hears in the distance the sounds of the Hebrews preparing for their departure, she declares that she must go and join them. In despair, Aménophis impiously defies her God, whilst Anaï implores him not to challenge God’s awful power. Furious, Aménophis intends to make his power over his Hebrew slaves felt, and goes into Moses’s tent to tell him this, but Anaï remains firm in her purpose: although realizing that her refusal will bring grief upon her people, she is ready to die with them.
On the point of leaving, the Hebrews are offering hymns of praise to God. On this day of rejoicing she alone seems to be unhappy: she begs God to help her to tame the worldly love that torments her, and her mother Marie prays with her. Moses and Eliézer emerge from their tent, full of anger against Aménophis, whom they threaten with the wrath of God. The Prince tries to have Moses arrested by the guards, but the Hebrews prepare to defend him. Drawn to the scene by the uproar, Pharaoh appears, surrounded by his royal suite; angrily he countermands his order to free the Hebrews and orders Moses to submit himself to his power. Moses urges the King not to awaken the wrath of God, to whom he appeals to give a demonstration of His power. Amid the general terror, the sky grows dark and the earth begins to shake: trees fall down and the great pyramid collapses, turning into a volcano which is seen to submerge the entire plains of Memphis in molten lava.
Inside the royal palace of Memphis. The darkness into which the whole of Egypt has been plunged has caused everyone to despair. Pharaoh finally decides to send for Moses and, to the anger of Aménophis and to the relief of Sinaïde, promises him that if he can restore the light the Hebrews will be set free. Moses prays to God and soon, to the general amazement, the darkness is dispersed: each in his own way is struck by God’s power.
True to his word, Pharaoh proclaims that the Hebrews may depart that very evening. It is in vain that Aménophis tries to oppose his father’s edict: his consternation increases when Pharaoh, to mark the day by celebration, invites everyone to the Temple of Isis, where he intends to formally announce his son’s forthcoming marriage to a Syrian princess. Aménophis is in despair, but dares not reveal to his father the true reason for his unhappiness. In any case, he does not intend to give up Anaï.
Sinaïde tries repeatedly to make him return to dutiful behaviour, but the Prince can think of nothing but his love for Anaï and his desire to be revenged upon Moses. In the distance choirs are heard inviting people to the Temple: Aménophis finally lets it appear that he has yielded to his mother’s persuasion.
The desert near the Red Sea. Aménophis has had Anaï carried off by force, and is now taking her to Moses with the intention of renouncing his claim to the throne in order to be able to stay with the woman he loves. But Moses declares that Anaï herself must choose between her family, her people and her religion on the one hand and her lover on the other. For all her suffering, Anaï chooses to remain faithful to her people and her God, praying to Him to restrain the mad passion of her lover. Aménophis is overwhelmed: in his fury, he exclaims that in any case the Hebrews have very little hope, as Pharaoh is marching against them at the head of his army. All are in despair, but Moses tells them to trust in God.
The shores of the Red Sea. Moses and the Hebrews invoke God’s help. Whilst they are praying, their chains miraculously fall off. However, a threatening rumble announces the imminent arrival of the Egyptian soldiers. Moses courageously advances towards the sea, and begins walking upon the surface of the water, inviting his people to follow him: all follow his example. The Egyptians rush after them, but a terrific storm bursts over their heads and they are drowned beneath the waves. When the storm dies down, the Hebrews are seen in safety on the opposite shore: Moses and his family sing a hymn of thanks to the Lord.