Captain Martin from the police's child department and his colleague Kraus are called to Znojmo to help solve a case regarding stolen toys found in the town's subterranean passages. The members of the local police department are convinced that the thieves are the well-known "customers" from the Znojmo elementary school, pupils Exner and Mandlík. Martin, however, has doubts about these culprits. These doubts grow even stronger after the local self-service shop is robbed and the local tobacco store reports that it is missing a lot of imported cigarettes. Martin questions the children, inspects both shops and searches through the underground.

The summer holidays are approaching and the children are bragging about all the exciting places they are going to visit during the long-awaited two months of summer. Only Tonda is not looking forward to staying in Prague over the holidays. He brightens up after his father announces that the boy can visit some friends of the family in the village of Petipas.