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.