Danish brilliant pre-teen student Nis uses his summer holiday to learn Latin so he can properly study the Knights Templar inheritance on Bjornholm island from sources. His friends Mathias, a dumb jock, and Kathrine, who only comes to spend time wear her dad, who really wanted a boy, prefer childish fun, but get dragged in his quest for a treasure. Mats's strength, Kathrine's connections and even Nis's bratty kid sister's blunt remarks contribute to find out about two brotherhoods, heirs of 12th century factions in the Templar order and deal with danger.

The snake crown is stolen (or taken back) from the Knight Templars by the earth worshippers of Crete; the three kids try to retrieve it on behalf of the Knight Templars.

In 1361 the Danish King Valdemar's troops approached the city walls of Visby, Gotland. Meanwhile, a young woman named Sigrid was leaving tracks all over the place leading to the whereabouts of a great treasure.She was able to fulfill her mission before being captured and immured alive in a tower, as punishment for not revealing the whereabouts of the treasure. Currently Kathrine, 14, and his three friends have to follow the tracks of 700 years ago during the Medieval Week celebration in Visby to rescue his father, who has been kidnapped, and find the lost treasure. But the fearful and dangerous Black Brotherhood lies in wait.