Kitty Fane is unhappily married to a bacteriologist working for the British government in Hong Kong. She imagines that the Assistant Colonial Secretary with whom she has been carrying on an affair will marry her, if they can both procure divorces, but he seems content to keep on in secret. When her husband Walter discovers her infidelity, however, he demands that she accompany him into a Chinese city beset by cholera.
It's an act of desperate, unimaginable cruelty. Walter--brooding, unsociable, bookish--wants to kill Kitty, and his anger is such that his own life is acceptable collateral. If you imagine that the cholera-stricken Chinese will offer Kitty an opportunity to recognize the shallow vanity of "society" and find redemption by caring for others, you are of course correct. The cholera epidemic awakens Kitty to the reality of death, though this awakening also gives her valuable perspective on her infidelity:
After all they had gone through, when they were living amid these scenes of horror and desolation, it seemed inept to attach importance to the ridiculous act of fornication. When death stood round the corner, taking lives like a gardener digging up potatoes, it was foolishness to care what dirty things this person or that did with his body.
Redemption comes in the form of an order of kindly Catholic nuns that care for sick and orphaned children; Kitty's only consolation comes from joining them in their duty. Much of The Painted Veil proceeds according to the progression of moral development you might expect; it defies expectation perhaps only in the sense that the marriage, and Walter, are irrevocably broken before their excursion and cannot wholly be saved in any sense. Maugham is careful not to oversell Kitty's moral development; her new compassion and awareness leave her a better person but also more isolated than ever.
But ultimately The Painted Veil failed to retain my imagination after it ended. Maugham traveled throughout Southeast Asia, but his China rarely seems anything but an Englishman's view, described as through the curtains of a sitting chair. Perhaps this makes sense, given Kitty's perspective--she is sheltered, for the most part, from the Chinese and from the devastation of the cholera--but it makes it difficult, I find, to comprehend the causes of her growth. I wonder what the novel would have been like if Kitty had been able to spend more time within the city walls.