At an old château, the custodian spices the routine commentary for visitors on a sightseeing tour with a story about murder and robbery which occurred at the château in 1923 during its restoration. The château manager was killed and thirty-three small diamonds were stolen from old weapons and a crucifix. When the visitors leave, the custodian, who doubles as a painter, dusts the exhibits. The glass of a weapons case breaks and falls out and the painter finds one of the diamonds. The fingerprints of a child are found on the glass and the investigation is thus entrusted to Captain Martin, the head of the police's child department.