Gold

Sam and Annabel are on vacation with Annabel's parents in Italy. While visiting the small hilltop town of Civita, they hear rumors of looted gold from World War II buried somewhere in the town's network of underground tunnels and caves. Once again the two friends cross paths with their old nemesis, Humphrey Battleford, but he is not the only one in pursuit of the gold. An intimidating man named Kurt, the grandson of a ruthless Nazi, is also snooping around.
After Annabel is kidnapped, Sam must solve the mystery of the hidden treasure to save his friend. This is the fourth mystery featuring Sam and Annabel, after Stolen, Bones and Lost.