Holger travels to Iceland to try to reach the place where Gjallarhornet is supposed to be before von Troppeln. Holger reaches the large and terrifying lava area Dimmuborgir. Here the final battle will take place.
In the final episode, Holger and Mortensen visit Sweden's three largest cities and their most famous buildings and places in order to finally solve the riddle.
After The order of the Black Sun steals the map, Holger and Mortensen learn that Mary Diana is on an island. Instead of aiming to take the map, they try to find and save Mary Diana.
Holger Nilsson meets the temperamental engineer Mortensen. Together they set out on a raging treasure hunt through Sweden to find the seven keys to Mary Diana Graaf's mythical laboratory.
To solve the riddle, they follow the Norrland coast. They drive through the breathtaking landscape of the High Coast, become shipwrecked on the island of Högbonden and climb high up to Slåttdalsskrevan.
A terrible enemy chases Mortensen and Esmé, who flee for their lives to the south. When everything seems to be working out, the situation changes dramatically and nothing goes as planned.