Northumberland is a pagan country where the King, Alla (based on Chaucer's understanding of the historical Ælla of Northumbria) eventually converted to Christianity. His evil mother intercepts and falsifies a letter between the couple, which results in Constance returning to Rome with their son. The King goes to Rome and in the end the couple return to Northumberland. Alla dies shortly after, and the baby boy becomes the King.
The tale is based on a story within the Chronicles of Nicholas Trivet but the major theme in the tale, of an exiled princess uncorrupted by her suffering, was common in the literature of the time. Her tale is also told in John Gower's Confessio Amantis, and both are similar to the verse Romance Emare.
Although Chaucer receives some praise and also criticism from his own character with favourable mentions of The Book of the Duchess and The Legend of Good Women; in the Man of Law's prologue he seems to spare most of his opprobrium for John Gower. Two of the tales which he dislikes, Canace and Apollonius of Tyre, involve incest, as did the some versions of the story. Chaucer based this tale on the Nicholas Trivet story from his Chronicle. Gower though had recorded all these stories. Chaucer is, perhaps, with friendly banter, trying to goad his friend and fellow writer into a storytelling challenge.
The tale is meant to be a morally uplifting story and is similar to hagiography, or stories of the saints' lives, which were common popular literature of the time. Custance as her name suggests is constant to her Christian religion despite the attacks and testing it receives from the pagans and heathens she meets on her travels. The Man of Law tells his story in a pompous over-blown style as if he is defending Custance in a court of law. He also uses many rhetorical figures, taken straight from the manuals of rhetoric of the day, to emphasize Custance's noble character—as well as the teller's lawyerly—and state her case.
The various manuscripts of the tales differ on the structure of the tales with some containing the Man of Law's epilogue and others not. In the epilogue, the host invites the Parson but he is interrupted before he can begin and a different speaker tells the next tale. The Summoner, the Squire and the Shipman are listed as interrupters in the different manuscripts but it is the Shipman whose character best matches the rude remarks although the mention of his "joly body" sounds closer to something the wife of Bath may say. What it probably shows is that Chaucer had not fixed his overall plan. There are also hints, with his claim he will talk in prose despite rhyming throughout, that the Man of Law originally told the Tale of Melibee before he was assigned Custance's tale late in the composition of the tales.