Katrine, Nis and Matias spend there summerholiday on the island Bornholm.
They are interested in old stories and myths and when they find out that the myths might be true they try to uncover the mystery.

Nis's little sister blackmails them into tacking her along, so all four of them go on a dangerous adventure to search for an ancient treasure which has been described in the myth: The Treasure of the Knights Templar.