Amun-her-khepeshef is the son of Pharaoh Rameses II, the grandson of Seti I and Tuya, and the adoptive nephew of Moses and Tzipporah.
He was first seen when Moses returns from the Midian desert because Moses had to tell Rameses that God came to him and that Rameses must let the Hebrew slaves go. Moses was delighted to see Rameses and his position as the pharaoh and to see his son, although Amun-her-khepeshef shows some dislike towards his adoptive uncle, possibly because he was technically a stranger to him, especially when Hotep and Huy upstage Moses with their own snake transformation. After Moses told Rameses God's message, Rameses did not understand and when Moses showed the power of the Lord God through a miracle of his staff turning into a snake, Rameses had his magicians Hotep and Huy duplicate the miracle (although their snakes were swallowed by Moses' snake and his serpent returned to a staff). After that Rameses took Moses to his throne room, and thinking that God's message was a joke, asked Moses what his return was really about. Moses then showed Rameses the Hebrew slaves and asked him what he saw and Rameses misinterpreted it and said it was a greater kingdom than that of his father while Moses said it was a kingdom made from slaves. Rameses said that he couldn't change what Moses saw and that he is responsible for his father's crown. Moses told Rameses that his father's hands bore the blood of thousands of innocent children and like his father Seti, Rameses saw the Hebrews as insignificant slaves while Moses finally revealed they were his people who were suffering as slaves.
Rameses was shocked when Moses revealed that he returned only to free the Hebrews. His heart is hardened when Moses gives back the ring that Rameses gave him some years earlier. Rameses then rises from his throne and says he doesn't know Moses' God and he will not let his people go free. Moses pleads with Rameses to listen, but Rameses says he won't be the "weak link in the chain" (a phrase harshly used by his father) and tells Moses his people's workload has been doubled. Later, when Moses has the confidence to face Rameses and by God's power unleashes the First Plague of Blood on the Nile River, Amun-her-khepeshef at first looks on excitedly at Moses’ would-have-been arrest until the river turns to blood and is shocked by the Plague while his father Rameses is unconvinced by it. Further in the movie, as the Plagues on Egypt come, Amun-her-khepeshef is seen with Rameses when the Fourth Plague, wild animals came, hiding himself with his father from the Plagues.
After the next five plagues, when Rameses is sharing a moment of brotherhood with Moses, Amun-her-khepeshef calls for his father and says that he is frightened because it is so dark and Rameses comforts him. When Moses warns Rameses that one last plague is coming and how Rameses threatens to kill the firstborn children of Israel, Amun-her-khepeshef learns the terrible truth of his grandfather Seti and backs away in fear. At midnight, the Tenth Plague comes as God sends the Angel of Death to smite every firstborn child in Egypt, but the firstborn of the Hebrews are safe since Moses told them to mark their doorposts with lamb's blood. Amun-her-khepeshef's and the rest of the Egyptian firstborns lives ate taken by the Angel of Death. After placing his son on a table and covering him with a cloth for burial, Rameses finally tells Moses that his people have his permission to leave Egypt. After leaving, Moses weeps for his dead nephew and for all the pain he caused his foster brother and his former home.