Having rid themselves of the threat of Magon the Olmec, the children are hot on the heels of Ambrosius who has opened the city of Ophir thanks to his medallion. A bitter fight for the city's treasure begins: the stone of Ophir, also known as the philosopher's stone. Meanwhile in Chambord, Nostradamus shows Tao the treasure of Chambord: a double-helix staircase designed by his master Leonardo da Vinci. But what secret does it hide?
