Olivier, 13 years old, moves with his parents to the countryside after his mother, a history and geography teacher, is transferred to Arville, an old town that prides itself on having a commandery that traces back to the Knights Templar and their preceptor.

While walking his friend Thomas’ dog, Olivier watches heavy work in a neighboring property and notices, horrified, that a shovel falls into a huge hole. Curiosity leads Olivier to go back to his neighbors’ garden and, to his great astonishment, uncovers with the shovel some old chests decorated with a kind of logo representing two riders on the same horse.

The adventure could have begun here, but because Olivier went on the neighbors’ property without permission, his strict father punishes him: Olivier is grounded for all of summer vacation and will not be able to leave the house.

Fortunately, Amanda, who he is secretly in love with, lives across the street and promises to help him.