Diane Allenby, dependent cousin of the wealthy Moreland family, rises from her sick bed after nearly dying of scarlet fever to find her beautiful black hair cut short and her selfish aunt and uncle expecting her to chaperone their daughter Louisa during her debut season in London.
Because she loves the beautiful blonde cousin she has looked after the past ten years, Diana is willing to do this, particularly wanting to protect her from the dashing Robert Welford, who has promised his mother he would choose the most beautiful debutante of the season as his wife.
Robert, who does not believe in love, courts Louisa through many social eventa, all while battling the chaperone who does not think he is good enough for her cousin. The second time he meets Diana, Robert makes a wager with her that he will marry her cousin before the season is over.
Diana takes this threat seriously and makes it her business to stop this from happening.