The beautiful young Theodora Fitzgerald belongs to a family of noble lineage whose fortunes have waned and who have lived in near poverty for most of her life. The book begins with her arranged marriage to Josiah Brown, a nouveau-riche Australian in his fifties. The marriage was contracted for convenience: Josiah simply wants a pretty and aristocratic wife to improve his standing in society, and the Fitzgerald family are in need of Brown's financial resources. Theodora only agrees to the marriage for the sake of her father and sisters.
Immediately after the wedding, Josiah falls ill. Theodora proves a dutiful and capable wife, and attends to her husband's every need, though she is secretly very unhappy. After a year of marriage, Josiah is well enough to visit Paris, where Theodora sees her father, Dominic, again for the first time since her wedding. She is thrilled to observe that at least he is receiving all the benefits she'd hoped to bring from her sacrifice: he now runs in aristocratic circles and is courting a wealthy American widow, Mrs. McBride. Theodora attends several social outings with her father, and at one dinner is introduced to Hector, Lord Bracondale. Theodora and Hector hit things off splendidly, and soon fall in love. Mrs. McBride is aware of Theodora's unhappy marriage, and seeing the situation she sympathetically arranges for Hector and Theodora to spend time together as often as possible. One day while Theodora and Hector are being chauffeured back to Paris after an outing at Versailles, the two indulge in a romantic encounter in the back of the car. Full of guilt thereafter, the two conclude they must behave themselves from now on and must no longer pursue each other romantically; they will, however, continue to be friendly to one another any time future social obligations might cause them to meet.
Hector at this point is terribly in love with Theodora, and though he tries his best to live by his promise to her, he still goes out of his way to see her and to secure invitations to all the same gatherings that she attends. He fantasizes about marrying her and makes sure to introduce her to his mother and to his sister. However, Theodora's status as a newcomer into society, and the obvious favor that Hector pays her over other eligible women who desire his hand, causes ire and jealousy to be directed her way. Rumors begin to spread, and several people believe Hector and Theodora to be lovers. Morella Winmarleigh, a spurned candidate for Hector's hand, particularly sets out destroy Theodora. She maliciously switches a letter Theodora had written to Hector with another letter meant for Josiah. Meanwhile, without anyone else's knowledge, Theodora and Hector have concluded that they cannot attempt to remain friends any longer their love is too strong and so they must agree to never see each other again.
The next day, Josiah receives Theodora's letter meant for Hector: the contents amount to Theodora asking Hector never to see her again, even though the two of them could be very happy together, because it is her duty to instead attend to the happiness of her husband Josiah. Josiah realizes for the first time how he has stood in the way of Theodora's happiness, and resolves to do his best to make her happy from now on. He forwards the letter to Hector and requests he never allow Theodora to learn of the mix-up. The next several months pass with Theodora and Josiah both trying their best to make the other happy, even while both are secretly miserable. Both begin to suffer from ill health. Ultimately, Josiah dies; eighteen months later, Mrs. McBride (now married to Dominic Fitzgerald) throws a picnic at Versailles to which both Theodora and Hector are invited. The book ends with the couple reunited, in a state of "passionate love and delirious happiness."
Output: Who switches Theodora's letters?