As a group of kids, whose families have been kicked out of their Marseille neighborhood, are about to leave forever, and just as they're about to pack up their dreams they find a mysterious box containing clues to a hidden treasure. Hoping to save their homes, they seek the help of Marius, the so-called "King of Marseille"? a notorious tour guide with a flexible sense of honesty, who knows the city by heart. As the kids and Marius discuss the treasure, they are overheard by Verna, whose three brothers happen to be the Katsopoulos gang, small-time crooks specialized in burglaries and obsessed with an ancient quest for a mythic gem. When she tells them what she heard, the brothers believe this gem may well be what Marius and the kids have stumbled upon. Seeing an opportunity to get out from under their authority, Verna turns against her brothers and joins Marius and the kids on a madcap, danger-filled adventure. With the Katsopoulos gang hot on their trail, the chase leads them down a mysterious underground staircase and into a breathtaking, booby-trapped treasure hunt for an extraordinary secret.