Maugham uses a third-person-limited point of view in this story, where Kitty Garstin is the focal character.
Garstin, a pretty upper-middle class debutante, squanders her early youth amusing herself by living a social high life, during which her domineering mother attempts to arrange a "brilliant match" for her. By age 25, Kitty has flirted with and declined marriage proposals from dozens of prospective husbands. Her mother, convinced that her eldest daughter has "missed her market", urges Kitty to settle for the rather “odd” Walter Fane, a bacteriologist and physician, who declares his love for Kitty. In a panic that her much younger, and less attractive, sister, Doris, will upstage her by marrying first, Kitty consents to Walter's ardent marriage proposition with the words, "I suppose so". Shortly before Doris’s much grander wedding, Kitty and Walter depart as newlyweds to his post in Hong Kong.
Just weeks after settling in the Far East, Kitty meets Charlie Townsend, the Assistant Colonial Secretary. He is tall, handsome, urbane and extremely charming, and they begin an affair. Almost two years later, Walter, unsuspecting, and still devoted to his wife, observes Kitty and Charlie during an assignation, and the lovers, suspecting they’ve been discovered, reassure themselves that Walter will not intervene in the matter. Charlie promises Kitty that, come what may, he will stand by her. Aware that the cuckolded Walter is his administrative inferior, Charlie feels confident that the bacteriologist will avoid scandal to protect his career and reputation. For her part, Kitty, who has never felt real affection for her husband, grasps that, in fact, he is fully aware of her infidelity (though he initially refrains from confronting her) and she begins to despise his apparent cowardice. She discerns, however, an ominous change in his demeanour, masked by his scrupulous, punctilious behaviour.
Walter suddenly confronts Kitty with an ultimatum: She must either accompany him to the Chinese interior to deal with a cholera epidemic, risking death, or he will file for divorce, with the proviso that he will allow Kitty to divorce him if Dorothy Townsend agrees to divorce Charlie and Charlie agrees to remarry Kitty immediately. Kitty goes to see Townsend who refuses to leave his wife. Their conversation, when she realises he doesn't wish to make a sacrifice for the relationship, unfolds gradually, as Kitty grasps Charlie's true nature. She is surprised to find when she returns home that Walter has already had her clothes packed, because he knew Townsend would let her down. Heartbroken and disillusioned, Kitty decides she has no option but to accompany Walter to the cholera-infested mainland of China.
At first suspicious and bitter, Kitty finds herself embarked on a journey of self-appraisal. She meets Waddington, a British deputy commissioner, who provides Kitty with insights as to the unbecoming character of Charles. He further introduces her to the French nuns who are nursing, at great personal risk, the sick and orphaned children of the cholera epidemic. Walter has immersed himself in the difficulties of managing the cholera crisis. His character is held in high esteem by the nuns and the native officials because of his self-sacrifice and tenderness towards the suffering children. Kitty, however, remains unable to feel attraction towards him as a man and husband. Kitty meets with the Mother Superior, an individual of great personal force, yet loved and respected. The nun allows Kitty to assist in caring for the older children at the convent, but will not permit her to engage with the sick and dying. Kitty’s regard for her deepens and grows.
Kitty discovers that she is pregnant and suspects that Charles Townsend is the father. She answers Walter’s inquiry by stating, "I don’t know". She cannot bring herself to deceive her husband again. Kitty has undergone a profound personal transformation. Soon after, Walter falls ill in the epidemic, possibly through experimenting upon himself to find a cure for cholera, and Kitty, at his deathbed, hears his last words.
She returns to Hong Kong where she is met by Dorothy Townsend, Charles's wife, who convinces Kitty to come to stay with them, as Kitty is now mistakenly regarded as a heroine who voluntarily and faithfully followed her husband into great danger. At the Townsend house, much against her intentions, she is seduced by Charlie and makes love with him once more despite admitting he is vain and shallow, much as she once was. She is disgusted with herself and tells him what she thinks of him.
Kitty returns to Britain, discovering en route her mother has died. Her father, a moderately successful barrister, is appointed Chief Justice of a minor British colony in the Caribbean and she persuades him to allow her to accompany him there, where she intends to dedicate her life to him and to ensuring that her child is brought up to avoid the mistakes she had made.