While their parents are busy shopping in a department store, Tiennot, who would rather chase girls, buys the complicity of Bébert, his little brother. In the train on the way back home, Tiennot, infuriated by his whims, leaves Bébert in the last car, which happens to be separated from the train and headed in another direction. Panic in the family: where on earth is Bébert? In fact, the little rascal has been taken in by the station master of Verneuil L'Etang, where he has a great time wreaking havoc in the SNCF network. A good-natured inspector is called on the spot and Bébert is finally reunited with his family.