1. Cahills vs. Vespers:
In the Cahill family, the secret is centered around the long-standing rivalry between two competing branches: the Cahills and the Vespers. For centuries, these two factions have been competing for power and influence, trying to find the 39 Clues, which are special objects with immense power. The secret lies in the knowledge of the final resting places of the 39 Clues and the ability to wield their combined power.
2. Cahill Family Secrets:
Beyond the rivalry, each branch of the Cahill family has its own set of secrets and hidden agendas. These include personal mysteries, hidden histories, and family legacies that individual Cahill members discover throughout the series. These secrets often revolve around the lives and actions of past Cahill ancestors, revealing crucial information about their choices, motivations, and impact on the family's destiny.
As the protagonists, Amy and Dan Cahill, progress in their quest for the 39 Clues, they uncover various secrets about their family and the hidden history of the Cahills. These secrets drive the plot, shape the characters' journeys, and build up to an explosive climax as the final truth about the 39 Clues is revealed.