Honzík and Martinek have learned something amazing from grandpa Koudelka: Apparently, the rocks will open on the Day of Pentecost and whoever was born on a Sunday can go into them to find a treasure! Honzík just happens to have been born on a Sunday, so he convinces Martinek to head out at night to the rocks in the forest. The forest is scary at night, but when the boys see a gap between the boulders, they boldly go inside. But an owl in the dark cave scares them and the boys get lost.