The Secret of the Forest Cabin

Vika and Lika are going to a summer camp, where they attend various interest clubs and get to know other kids.
One day, the selanites, unremarkable minerals, disappear from the young geographers' classroom.
At the same time, strange things start happening in the camp: a ghost appears, and terrifying screams can be heard from the forest. The kids start an investigation. They are curious to find out whether these minerals are truly worthless or if there is some secret hidden within them.