“Our uninhabited island” - this is what Masha, Ilya and Yegorka called an old dovecote, which they stumbled upon one day in one of the Moscow courtyards. The guys began to disappear there all day long, even setting up a real hiding place to pass notes to each other. But it turns out that they are not the only ones who know this place. One day Masha and the boys find in their hiding place... jewelry that someone left there while they were away. What could this mean? Did the thief hide the loot? Or, on the contrary, is someone trying to preserve their own property?.. We need to find out - trace, detect, identify the criminal. The operation was a success. True, there were several villains. And against them are three teenagers...