Shortly after encountering the earnest and socially awkward bacteriologist Walter Fane at a lively party, the vivacious, yet vain London socialite Kitty Garstin hastily enters into a loveless marriage with him, largely due to her domineering mother's influence. Following a romantic honeymoon in Venice, the couple relocates to Shanghai, where Walter is stationed at a government lab dedicated to combatting infectious diseases.
In this new environment, Kitty crosses paths with the charming but married British vice consul, Charles Townsend, igniting an illicit affair that she believes will fulfill her desires. However, when Walter uncovers the painful truth of his wife's infidelity, he resolves to punish Kitty by threatening to divorce her on the grounds of adultery unless she agrees to accompany him to a remote village in China. Here, he has volunteered to assist in combating a devastating cholera epidemic that has gripped the area.
Desperately, Kitty pleads for a quiet divorce, but Walter concedes to this only if Townsend will leave his wife, [Dorothy], and marry her. When Kitty presents this precarious option to her lover, he adamantly refuses, leaving her no choice but to embark on this arduous journey with Walter. Their trek to the mountainous region spans two grueling weeks, a journey that could have been expedited considerably by river travel. However, Walter is determined to make Kitty's experience as uncomfortable as possible, further deepening the rift between them.
Upon their arrival in the impoverished village of Mei-tan-fu, Kitty is dismayed to find they are forced to live in near squalor, isolated from the outside world, save for their amiable neighbor Waddington, a British deputy commissioner who enjoys a more opulent life with his young Chinese partner. As Walter and Kitty drift further apart and communication dwindles, Kitty finds herself alone for long stretches of time, particularly after engaging in volunteer work at an orphanage run by French nuns.
In this new atmosphere, Kitty is surprised to discover that Walter has a heart for children. As she becomes more involved in the community, she sees a side of Walter she had previously overlooked; his selflessness and compassion start to come to light. In turn, Walter begins to view Kitty as more than just a shallow figure, sparking a gradual transformation in their relationship. Their emotional barriers begin to erode, leading to an unexpected blossoming of their marriage.
However, as fortune would have it, Kitty soon finds herself pregnant, leaving her in a tumultuous state of uncertainty regarding the true identity of the father. Despite this confusion, Walter, now rekindled in his affections for her, reassures her that the child's parentage is inconsequential in the grand scheme of their life together.
Just as the local cholera crisis starts to wane, a surge of refugees inundates the area, compelling Walter to establish a camp to assist the ailing population. It is during this time that he contracts the very disease he has been battling, and Kitty, filled with a sense of duty, nurses him back to health. Tragically, Walter succumbs to the illness, leaving Kitty devastated and grappling with overwhelming sorrow.
Fast forward five years later, while out shopping in London with her young son, appropriately named Walter, Kitty serendipitously encounters Charles Townsend once again. His casual suggestion to meet up triggers a moment of realization as he inquires about her son's age—an indication that he might indeed be the child's father. However, Kitty resolutely dismisses Townsend's advances and walks away, responding to her son's innocent question about Townsend with a simple, yet weighty, "No one important."
