Kitty Fane is a beautiful, vain, upper-class British woman unhappily married to Walter Fane, a bacteriologist working in Hong Kong. She is conducting an affair with Charles Townsend, the Assistant Colonial Secretary, and the two are in her room when the knobs to her door mysteriously turn from the outside.
She worries that her husband may have discovered her affair, but then she recalls their loveless match. She married Walter after several years of unsuccessfully seeking a match who would win her mother's approval; eventually, her homely younger sister Doris was about to be married, and Kitty hastily accepted Walter's proposal of marriage so that she would not be single at her little sister's wedding. The outgoing Kitty and the reserved Walter make a horrible match.
Walter is silent and pale for several days, and then gives Kitty an ultimatum. She will accompany him on his expedition to fight a cholera epidemic in Mei-tan-fu, a city in southern China, or he will bring her to court for her infidelity. Kitty is horrified, and certain that the cholera epidemic will mean her death. Walter gives her another out: if Charles agrees to marry her, he will allow her to divorce him and she can stay in Hong Kong. Kitty, delighted that she will finally get the chance to be with her beloved Charles, immediately confronts her lover with this proposal. Charles refuses to divorce his wife Dorothy, and Kitty, heartbroken, agrees to go with Walter to Mei-tan-fu.
The cholera epidemic in the city is severe, and scenes of death are all around. Kitty finds a friend in Waddington, the Deputy Commissioner of Customs in Mei-tan-fu, a short and rather ugly little man who is humorous, kind, and insightful. He introduces her to the nuns of Mei-tan-fu, French Catholics who inspire Kitty with their aura of peace and devotion. She begins to assist the nuns with their work, caring for orphans.
Kitty's work at the convent changes her personality. She sees her former acts of selfishness and attempts to make amends for them, apologizing to Walter for her affair. The situation is complicated further when she realizes she is pregnant.
Walter dies of cholera, but not before Kitty offers a final apology. Not long after, she heads back to Hong Kong, where she is received by none other than Dorothy Townsend, Charles' wife. Dorothy invites her to stay at their home, tearfully praising her courage, and Kitty awkwardly accepts.
Charles is polite and distant to Kitty in public, but in private attempts to seduce her once more, and Kitty falls victim to his advances. Disgusted with herself and him, she flees back to England to the home of her parents. She receives several cold letters from her mother indicating health problems, and receives news of her mother's death shortly before reaching England.
Kitty visits with her father, a quiet man who is adjusting to a life where Kitty’s domineering mother does not badger him. He plans to take a legal post in the Bahamas, and Kitty begs to go with him. For the first time in her life, Kitty has an honest conversation with her father, and establishes a relationship with him. Kitty plans to raise her unborn daughter as a strong, independent woman.