In order to eradicate malaria, endemic in the marshes of the Mingrelia region, the authorities plan to drain a large territory that according to legend is the realm of "Uzhmuri", the Queen of Frogs and a deadly deity for intruders.

A pioneer group has set up camp in a mountainous village in Georgia. The pioneers are collecting medicinal plants in response to a call from the newspaper Pravda. Only little Leo, the son of a forester, stands aside from this work. One day, Leo accidentally sets fire to the grass that was being prepared for shipment. The pioneers and the workers put out the fire with great difficulty. But the leadership of the pioneer camp does not punish Leo. They believe that he needs to be reformed, so they enroll Leo in the pioneer group.