Former astronaut Perry Housman has invited Frank and Joe to Tunisia for the hundredth anniversary of the International Explorers Guild. There they join some of the most intrepid young men and women in the world -- trailblazers, mountain climbers, deep-sea divers. But of all the challenges the explorers face, none is more deadly than the killer now in their midst!

Thor Sonderstrom, who controls the money that can make or break an expedition, has been poisoned. Determined to unmask the murderer, the Hardys soon find that the site of the celebration, an ancient fortress, has turned into a perilous palace of intrigue. In a world of shadowy secrets and venomous hatreds, danger and death lurk in the most unexpected places!