Raised by an unloving father who told him he was actually a bastard, Gareth St. Clair, the second son of a viscount, has plenty of private demons to battle. Years later, after ironically becoming the heir to the St. Clair title when his older brother dies, Gareth still doesn't understand why his father, Lord St. Clair, hasn't publicly denounced his position in society.

His brother, always a champion of Gareth, leaves him a diary written in Italian. Unsure of what to do with the mysterious journal, he heads to his grandmother's house where he meets Hyacinth Bridgerton, youngest of the famed eight Bridgerton siblings. Though she is known for being outspoken and dramatic, there is something charming about her that keeps her in everyone's good graces. Fortunately for Gareth, Hyacinth can read Italian and, as she begins to decipher the text, clues to the mysterious St. Clair history begin to unravel.

Long ago, Gareth's grandmother hid jewels on the St. Claire estate, jewels that can replenish St. Claire coffers left bare by Lord St. Claire's gambling debts. As Gareth and Hyacinth begin their search, they soon realize that they're uncovering another treasure, a love infinitely more precious than any shiny bauble.