On April 7, 1820, the day before Miss Penelope Featherington's 24th birthday, Lord Anthony Bridgerton found his brother, Mr. Colin Bridgerton, in the gardens where the Featheringtons once lived. 

  

The illusion of a happy marriage between Colin Bridgertion and Marina Thompson-nee Bridgertion shatters when Lady Debling dies after giving birth to a lovely baby girl, prompting Lord Alfred Debling to begin a courtship with widow Lady Cressida Twombley. 

  

Filled with guilt and grief over losing Colin, Anthony makes a promise to the stars on his brother's behalf, vowing to protect the child as if she were a Bridgerton. 

  

Seventeen years after their deaths, Miss Harriet Debling is presented to Queen Victoria and enters Mayfair's society. Besides the gentle breed of eight and one is Lady Cressida Debling, a widow for six years. Recognizing the opportunity, the Viscount approaches the ladies. 

  

The Bridgerton family is so charmed by Harriet, who resembles Penelope, that they begin preparations to invite the young debutant to Audrey Hall during the off-season. 

  

While the family becomes enamored by her wit and intelligence, Anthony and Kate witness their third son, Colin, gazing at Harriet just as his namesake gazed at the previous Lady Debling, Penelope. 

  

To prevent history from repeating itself, the Viscount and his Viscountess seek ways for the pair to meet. With Lady Violet's clever schemes and Lady Eloise Crane's access to Harriet, the Bridgerton family unites to help Anthony keep his vow of protection. 

  

The news of this love match blooming causes Lady Cressida Debling to set her match for Harriet with Mayfair's rake. 

  

Is this truly where Penelope and Colin's tragic love story ends? 

  

Will Anthony and his family succeed in bringing Harriet and Colin II together? 

  

Find out in "Promise the Stars."