Egypt has been plunged into the deepest darkness, a punishment inflicted by God upon the subjects of Pharaoh, who has not fulfilled his promise to free the Hebrews from slavery and allow them to leave for the Promised Land. Terrified, the Egyptians beg their king to free them from the curse: so Pharaoh summons Moses, the leader of the Hebrews, and promises him that his people shall go free as soon as the sun shines once again upon his country. Although his brother Aaron advises him not to believe in Pharaoh’s promises, too often broken, Moses implores God to forgive Egypt, raises his rod and the darkness is dispelled. Moses and his people will be able to leave Egypt before nightfall.
Osiride, Pharaoh’s son, is secretly engaged to Elcia, a young Hebrew girl. Fearing to lose her, he tries to prevent the Hebrews from leaving, convincing Mambre, the high priest, to help him in this attempt. He therefore persuades him to encourage an uprising of the people against Pharaoh’s decision, which would rob the country of all its slave labour. Mambre considers Moses a charlatan and his miracles nothing but cheap tricks, like those that he himself was able to do in bygone days. Therefore he agrees to stir up revolt among the Egyptians. Elcia comes on in tears to bid a last farewell to her lover.
Mambre’s plots cause an infuriated mob to gather in front of the royal palace, calling loudly for the order of liberation of the Hebrews to be revoked. Pharaoh allows his son to persuade him to take back his promise once more, and sends Osiride to warn Moses that every Hebrew who tries to escape will be killed. This greatly disturbs Amaltea, Pharaoh’s wife, who is trying to protect the Hebrews because she herself has been secretly converted to their religion. The Hebrews, who believed that they were free at last, are desperate when they hear the news and Moses threatens more divine punishments for Egypt. Osiride then orders his soldiers to kill Moses, and only the entrance of Pharaoh prevents blood being shed. Pharaoh confirms his latest decision and Moses, raising his rod, calls a rain of fire from heaven.
To ward off the latest divine curse, Pharaoh orders the Hebrews to depart at once. He then calls his son to inform him of the happy conclusion of a treaty of marriage between him and the Princess of Armenia, and cannot understand why such a joyful piece of news is received so ruefully. Shortly afterwards Aaron tells Moses that Osiride has run away with Elcia and that he has had the guilty couple followed to find out where they are hiding. Moses asks Aaron to tell Amaltea of this and to go with her to find the lovers.
In the dark cavern where he has led Elcia, Osiride tells her of the cruel situation in which he finds himself because of his father’s plans for him. He proposes that they remain in hiding together, and lead a clandestine life in the woods. These happy fancies of the two lovers are interrupted by the arrival of the Queen, escorted by Aaron and her guards. In spite of the general disapproval, they refuse to be separated and Osiride announces that he intends to give up his claim to the throne. Meanwhile Pharaoh, fearing that once the Hebrews are free they will rush to support Egypt’s enemies, revokes his promise yet again. Furious, Moses declares that the heir to the throne and all the firstborn male children of the Egyptians will be struck dead by a thunderbolt from heaven. Moses is put in chains for this threat, and Pharaoh, wanting to save his son from the effects of the prophecy, summons the assembly of nobles, and declares that from now on Osiride will share the throne with him, and orders Osiride himself to pronounce the death sentence upon Moses. To the amazement of everyone present, Elcia now comes forward to reveal her engagement to Osiride and implores him to free Moses and let him depart with his people. She then begs him to accept his royal destiny and marry the Princess of Armenia, leaving Elcia to expiate her error with death. But Osiride will not change his mind: he gives the order that Moses should be executed immediately. Pharaoh and Elcia cry out in despair as Osiride falls, struck by a thunderbolt.