[For an introduction to Dame Frevisse, if you are new to these books by Margaret Frazer, see JM's review of The Servant's Tale.]
This story is set in 1449, the year in which the ineffectual English King Henry VI began to lose (or have lost for him he did nothing) all the territories in France that his predecessors (Edward I, Edward III, his son the Black Prince, Henry V) had insisted on asserting their claim to; and this forms the sub-plot, for here once again Dame Frevisse moves among the great of the land.
But the main story concerns the misfortunes of Cristiana, wife, then widow, of Sir Edward Helyngton, a knight in the service of the King. Before Edward dies, he warns her that his cousin Laurence, the villain of the piece, will try to steal away their lands and also their two young daughters, because one of the daughters must be made to marry his son in order to legitimise his claim to the land. Then Edward tells her and her brother, Gerveys, of a document that incriminates the Earl of Suffolk, de facto ruler of the country, in treasonable acts against the King. This she must use to force Suffolk to support her if Laurence, who is Suffolk's man, does abduct the two girls.
But within days of Edward's death, Cristiana is kidnapped and taken to a nunnery, with papers signed by Suffolk stating that she is both mad and wanton, and unfit to be a mother, that she is to be kept in the nunnery as a penitent (in effect as a prisoner on a punishment regime) and that Laurence is the official guardian of the two girls.
The nunnery, of course, is St Frideswide's, home of Dame Frevisse; and she just happens to be the cousin of Lady Alice, Suffolk's wife.
I must say that of all Margaret Frazer's characters, Cristiana is the one I most sympathised with - identified with, in fact. And the shocking ending really moved me.