The book tells the story of Millie Calloway, who works as a housekeeper for a wealthy Manhattan couple. She discovers strange rules and clues suggesting something is wrong and decides to help the lady of the house, Wendy, despite risking her own dark secret.

The book explores central themes like secrets and lies, trust and mistrust, and courage and self-sacrifice. These themes drive the plot forward and create constant tension and emotional depth.

Millie Calloway shows great courage and self-sacrifice by doing everything to help Wendy, despite risking exposure of her own secret. Her bravery and determination make her an inspiring and admirable character in the book.