The Traces Remain (1956)

This is a typical story from the cold war era A group of kids play on a quiet street in Sofia From a nearby window falls a key The helpful youngsters quickly return it to the owner but in the haste switch the keys The unintentional error disentangles a whole spy story The young detectives doubt the good intentions of the man they tried to help It turns out that the key is from a secret apartment where saboteurs are hiding The secret network is exposed to the authorities and the kids feel like heroes