Emil from Neustadt travels to Berlin to visit his grandmother; he takes 140 Mark with him which his mother is sending. On the train, the money is stolen by another passenger. In Berlin, Emil pursues the thief and meets a gang of boys of his own age who help him. Eventually they recover the money; the thief - who turns out to be a bank robber to whom the police had been wanting to talk in any case - is arrested, and Emil gets a big reward.
The quality of the video I watched was awful: jerky, and the sound was so poor that the English subtitles were a real help. If this film has not yet been carefully restored, it ought to be. It is great. The child actors are wonderfully lively and natural, and Fritz Rasp gives a delightfully sinister performance as the thief. Inga Landgut is great as Emil's cousin Pony Hütchen (a nickname that refers to her funny little hat). The dialogues are equally good. You might think that children addressing each other as 'meine Herren' ('gentlemen') would sound unnatural, but they don't. That is evidently how children were talking c. 1930.
And that takes me to another great aspect of 'Emil und die Detektive': You get a glimpse of Germany in the brief period between the wars when conditions were more or less 'normal' by the standards of Western Europe and the US. That does not mean that there was no hardship. Emil's mother is clearly struggling as a hairdresser, and the 140 Mark she sends to his grandmother are probably about as much as she would make in a month. What I mean is normal in the sense that the militarism of imperial Germany had abated (even the constabler in Neustadt is reasonably friendly, despite his pre-WWI appearance) while marching hordes of brown shirts were not yet in evidence. When the film came out that was already about to change. Nine years later the actor playing Emil (Rolf Wenkhaus) would join the Luftwaffe; he was killed in action off the coast of Ireland in 1942. Most others did not survive the war either.