The nun is not described in The Prologue, but she tells a saint's life history. She is the secretary of The Prioress and she relates the history of Santa Cecilia. Some scholars speculate that the tale is merely the second tale of the single Prioress but this idea is not widely held.
Santa Cecilia is the daughter of a Roman. She is engaged to be married to a man named Valerian. But Cecilia, hopes to remain a virgin all her life, and prays to God to make it so. On her wedding night, she offers to tell her new husband a secret. She tells him that she has an angel protecting her chastity and If her new husband trys to take her virginity, this angel will kill him. Valerian asks to see the angel as a token that he should trust her, but Cecilia tells him he can only see it if he becomes a Christian. So, she sends Valerian to be baptised by The Papa Urban.
When he came back, Valerian is able to see Cecilia's angel. It offers to grant one request of Valerian's. Valerian asks that his brother Tiburtius be brought to the true faith, and the angel assents. Finally, Tiburtius meets Pope Urban and is baptised too for can see the angel.
After a time, the Roman prefect Almachius declares that all people who refuse to adore at the shrine of Jupiter will be killed. So Valerian and Tiburtius refuse to adore and they are executed.
Almachius orders Cecilia brought before him. Furious, Almachius sends his executioner to kill Cecilia with a sword. Despite taking three strokes, however, the man is unable to kill her. Cecilia continues to live, spurting blood and with her head partially severed from her body, for three days.
During this time, Celia continually preaches to and teaches the people.