Young French archeologist, Dr. Claire Dorval, is drawn in 1967 to the then peaceful kingdom of Afghanistan to examine an engraved stone found in the Hindu Kush. In the process, she discovers a 23-century-old message from Alexander the Great concerning a reserve hidden for his Afghan Queen, Roxanne, in case something happened to the young conqueror.

The "riddle" relates to where the reserve was left, and the question of whether it's still there is a monumental one. Alexander inherited from ancient Persia some 3,000 tons of bullion and jewels worth more than 20 billion of today's...