The events of the Russian historical detective series "Rostov" unfold in Rostov in the 1920s. A former prisoner becomes a criminal authority and holds the entire city in fear. Meanwhile, Ivan Kozirev, nicknamed Kozir, who used to be a robber, becomes the head of the criminal investigation department. He has embraced Soviet power and is ready to serve it. The authorities have no doubt that only Kozirev is capable of eliminating crime in the city, as he is familiar with this world.