On 25 April 1605, the knight Milota was killed by the royal armourers while escaping from the castle of Temice. However, the robbed treasures were not found in his possession. Almost 400 years later, Jindra, Tomas and eighteen-year-old Alena spend their holidays at the castle with their grandfather, the castellan. The castle needs repairs, but there is no money for them. Tomas and Jindra decide to find Milot's treasure, which could help the castle. The visitors are guided by Alena and two temporary workers, Roman and Petr. Among the tourists, Professor Horák, who shows a suspicious interest in the castle's collections, attracts the boys' attention...

An adventure story about three boys who are at summer camp and soon notice that very strange and unexpected things are happening near the camp. And like proper boys, they decide to get to the bottom of all these mysterious puzzles. After a series of adventures and comical misunderstandings, the story's dénouement reveals that the boys have helped expose smugglers who are bringing refugees from the East across the nearby border and robbing them of their meager resources.