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....