Walking through London's West End after a night at the theatre, Amory Ames and her husband Milo run into old friends, the wealthy investor and former actor Gerard Holloway and his wife Georgina.
When Holloway invites them to the dress rehearsal of a new play he is directing, Amory readily accepts. However, she is shocked to learn that Holloway has cast his mistress, actress Flora Bell, in the lead role.
Furthermore, the casual invitation is not what it seems-he admits to Amory and Milo that Flora has been receiving threatening letters, and he needs their help in finding the mysterious sender.
Despite Amory's conflicting feelings-not only does she feel loyalty to Georgina, but the disintegration of the Holloways' perfect marriage seems to bode ill for her own sometimes delicate relationship-her curiosity gets the better of her, and she begins to make inquiries.