The Medusa plot

Thirteen-year-old Dan Cahill and his older sister, Amy, thought their quest for the 39 Clues was over. They were wrong. When distress calls come in from around the globe, they discover Cahills are being kidnapped by a shadowy group called the Vespers. Now, Amy and Dan have mere days to fulfill a bizarre ransom request or their captured friends will face dire consequences. The Vespers are playing to win, and if they get their hands on the Clues, the world will be their next hostage.