Kitty Fane is a beautiful young woman whose mother has raised her to make a suitable match. But Kitty refuses a number of suitors; several years pass and eventually she is reduced to marrying Walter, the colonial bacteriologist in Hong Kong. Walter is a shy and awkward man who loves Kitty passionately, but has no idea how to express it; Kitty is charming and socially adept, but vacuous. In Hong Kong Kitty engages in a yearlong affair with Charles Townsend, the assistant colonial secretary, and a married man whose celebrity potential far eclipses Walter's stolid scientific work. The novel opens when Walter discovers his wife's infidelity.

Kitty believes that Townsend is madly in love with her and prepared to divorce his wife and sacrifice his career to marry her. Walter, who suffers from a broken heart, gives Kitty an ultimatum--either Townsend must promise to divorce his wife and marry her, or Kitty must accompany Walter to a city in the interior where he has volunteered to go to fight the cholera epidemic. Townsend demurs; Kitty is crushed; and the desperately unhappy pair travels to the cholera-ridden city, where they move into the house of the newly-dead missionary.

There, Walter (who is also a medical doctor) sets to work, day and night, to institute public health measures and care for dying patients. Meanwhile, Kitty meets Waddington, the British consul, a cynical alcoholic, who is at heart a good and honest person; and the French nuns, who labor tirelessly to care for orphans and the ill. Impressed by the nuns' selflessness, Kitty begins to devote herself to assisting them and trying to understand their spirituality.

When he learns that Kitty is pregnant, Walter asks if it is his child; Kitty responds, "I don't know." This completes the destruction of Walter's heart, and he soon dies of cholera--presumably as a result of experimenting on himself to find a cure. Kitty learns that the nuns, the soldiers, and all the people of the city consider Walter a saint, who has sacrificed himself for their welfare. However, while Kitty has learned to respect her husband, she could never love him.

Kitty stays only briefly in Hong Kong before returning home to London. Shortly before her arrival, she learns that her mother, whom she believes is responsible for her (Kitty's) shallowness, has died. The novel ends with Kitty vowing to bring up her daughter as a strong and independent woman, and preparing to move with her father to the Bahamas, where he has recently been appointed Chief Justice.