In early 20th-century London, Kate Croy is secretly engaged to Merton Denscher, a journalist possessing all the qualities of an ideal husband except money. By chance, Kate befriends American heiress Milly Theale, whom she learns is suffering from a mysterious fatal illness. Kate, who truly cares for Milly, devises a scheme to maximize their combined assets: she encourages Merton to take an interest in Milly, to seduce her, and finally, to marry her. By lending her lover to brighten Milly's few remaining days, Kate intends to make him a rich widower whom she herself can marry. But such well-laid plans are not enough for the subtlety of the heart.