Book is in French It's the first day at a new school for Gabriel, Aria and Mamadou who wonder what will happen to their detective agency, The Three Musketeers, when they return to class. But when a class assignment leads them to explore the White Caverns, on the edge of the village of Hillsborough, they find themselves inexplicably catapulted into an underground world where they will be confronted with all kinds of creatures, each one more bizarre and dangerous than the others. Even stranger is this prophecy which predicts that the-three-that-are-four will come from the world above to save the Hollow Earth from the clutches of the foul Goroth. Are they really, as Princess Ysäne believes, the heroes we have been waiting for over a thousand years? And who are these mysterious doubles of people they already know? Like the morose Bouli, who is a carbon copy of Billy the Terror, the tough guy from the schoolyard. Not to mention the mage Jarnigus, whose name sounds oddly like that of their old friend, Professor Jarnigoine. So many riddles that our friends will have to solve if they hope to triumph over the dangers that await them.