Fourteen-year-old Jan and his companion Erling appear as extras in a film being filmed at Nordisk Film's studios in Valby. A mysterious burglary at the studio's fine mechanics workshop gives the two healthy boys the opportunity to act as detectives.
| Actor | Role |
|---|---|
 Henrik HuldJan | Jan |
| IIb LundtoftErling | Erling |
 Mimi HeinrichLis | Lis |
 Gabriel AxelInstruktøren | Instruktøren |
 Bent ChristensenInstruktørassistent | Instruktørassistent |
 Paul HagenProduktionsleder | Produktionsleder |
 Ib SchønbergInspektør | Inspektør |
 Asbjørn AndersenDirektør | Direktør |
 Axel StrøbyeRegissør | Regissør |
 Inger StenderNot listed | Not listed |
 Palle HuldNot listed | Not listed |
| EEinar JuhlNot listed | Not listed |
 Bjørn SpiroNot listed | Not listed |
 Henry NielsenNot listed | Not listed |
 Gunnar StrømvadNot listed | Not listed |
| JJørgen KunzNot listed | Not listed |
| IIb FürstNot listed | Not listed |
 Jessie RindomNot listed | Not listed |
 Henny Lindorff BuckhøjNot listed | Not listed |