A Sultan from Syria hears of Custance, the daughter of the Roman Emperor, who is said to be the goddess of beauty, youth, and virtue. The Sultan falls in love with her without meeting her and is willing to do anything to have her. Later, Custance travels alone on a boat to Syria to marry the Sultan, without knowing him.
"Thou hast slandered, guiltless."
The Sultan converts to Christianity in order to marry Custance. His mother is furious and plans to have the knights who converted into Christianity and her son killed during a feast. Her plan succeeds and sends Custance to Northumberland on a boat.
Custance arrives at Northumberland, and meets a constable and his wife, Hermengild, who take her in. Both convert to Christianity. A knight notices Custance and falls in love with her but Custance denies his love. Later at night the knight kills Hermengild and leaves the knife next to Custance. Custance is then blamed.
The King, Alla, judges Custance guilty but then a hand struck the knight and a voice was heard saying that he was the guilty one. After this occurred Northumberland converts to Christianity and Alla, the king, falls in love with Custance.
After Alla and Custance get married Alla goes to war. While he is at war Custance has a baby boy, named Mauricius. Alla’s mother, who does not like Custance, sends a letter to her son saying that Alla son is an evil creature, Alla replies that his wife and his son should be cared for but his mother doesn’t listen and sends Custance to Rome.
"So horrible a fiendish creature."
King Alla learns about what happen to Custance and sends his mother to be killed. Allah goes to Rome looking for Custance. A Senator plans a feast for Alla where Alla notices his child, Mauricius. When the feast ends, Alla and Custance reunite and they go back to Northumberland, but king Alla dies soon after. Mauricius,Custance son, becomes Emperor of Rome.