The Secret of the Forest Cabin

Vika and Lika go to a summer camp, where they participate in various interest clubs and meet other kids.
One day, some seemingly ordinary minerals disappear from the young geographers' classroom.
At the same time, strange things start happening in the camp: a ghost appears, and eerie screams come from the forest. The kids begin an investigation. They are curious whether these minerals are truly worthless or if there is some secret hidden within them.