It tells the story of the historically important, 14th-century love affair in England between the eponymous Katherine Swynford and John of Gaunt, Duke of Lancaster, the third surviving son of King Edward III.

By virtue of this connection, Katherine meets and marries Sir Hugh Swynford of Lincolnshire and gives birth to a son, Thomas, and a daughter, Blanchette.

[6] In 2006, Margaret Moser of the Austin Chronicle described the novel as "a glorious example of romance in its most classic literary sense: exhilarating, exuberant, and rich with the jeweled tones of England in the 1300s".

[6] Nick Rennison believes that the novel has retained its popularity "because of the skill with which [Seton] evokes the world her heroine enters and because Katherine herself remains such a powerful character".

Though subsequent non-fiction accounts of Katherine, including those by historians Alison Weir and Jeanette Lucraft, make clear that Seton's various speculations were partly and sometimes significantly incorrect, the novel does provide the reader with a reasonably accurate view of medieval England, life at court, and the lives of women in the 14th century, along with intelligent and sensitive glimpses of Chaucer, Katherine's brother-in-law.

In addition, Katherine is one of the few fictional texts that include writings by the Blessed Julian of Norwich, a character in the novel, who is considered one of England's greatest mystics and whose Revelations of Divine Love was the first book written in English by a woman.