The action occurs twelve years later on from the last novel, when Hannay, now in his fifties, is called by an old oath to protect the son of a man he once knew, who is also heir to the secret of a great treasure.
In the aftermath of twelve years, Hannay, now in his fifties, is compelled by an ancient vow to protect the offspring of a person he previously encountered, who also holds the key to a significant fortune.