The sanatorium guard was strangled. A pharmacist was thrown out of a window. Scientist shot dead. Before this, they all received envelopes with numbers 1, 2, 3 from the mysterious killer... The envelope with number 4 was received by a large businessman. Looks like he's the next victim. And there are also other envelopes. These people have only one thing in common - they are all childhood friends. So, the motives for the murder must be looked for in their past? This is the first thing that comes to Drongo's mind. But the famous detective knows from experience that the solution to a crime rarely lies on the surface. We need to dig deeper. And look for what the criminal tried to hide from view...