Told by a nun concerned only with spiritual matters, this tale tells the story of Saint Cecilia. The form of saints' lives popular in the Middle Ages is of less interest in modern times and as such, the tale is not as highly regarded as other parts of the Tales. It is among the best of rhyming saints' lives in English. The popularity of such tales mean there are numerous sources which Chaucer may have used including elements from Dante.
The lack of portrait for the second nun in the General Prologue have led some scholars to speculate that the tale is merely the second tale of the single nun or Prioress but this idea is not widely held. Its relationship to the subsequent Canon's Yeoman's tale is to offer a serious and worthy religious themed story before a much more irreverent view of contemporary religious behaviour of foolish alchemists.
A Roman prefect, Almachius, learns that Valerian and Tiburce are practicing Christianity, and has them summoned, telling them that they must make sacrifices to an idol of Jupiter or be put to death. They refuse, and are killed, but not before they convert Almachius's officer, Maximus, and several executioners. Upon learning of Maximus's conversion, Almachius has him beaten to death with a lead-tipped whip.
Almachius then calls for Cecilia to be summoned to him. At first, his ministers refuse, having been converted to Christianity. However, he is ultimately successful in bringing her before him. He demands that she make sacrifices to the idol of Jupiter, and she refuses, calling him foolish.
Angry, Almachius sentences her to be boiled alive, but miraculously, the cauldron of boiling water does her no harm, and she sits quite comfortably in it for an entire day. At the end of the day, Almachius orders her executed by other means, and an executioner tries to behead her, but is not able to cut completely through her head in three strokes, and is bound by law not to attempt a fourth. Miraculously, despite having her throat cut, Cecilia continues to live, preaching Christianity, for three more days.