One dark winter evening, the young tsar Saltan was lucky enough to find himself a tsarina from a simple Russian family. He was so impressed by the speech of the young charmer that he married her the same evening, and took her two sisters with him to the palace as servants. However, the treacherous girls do not share the happiness of their luckier sister and begin to plot. Saltan leaves for a long military campaign, during which the young queen gives birth to his son. The evil sisters write a dispatch to the tsar, where they brazenly lie about the child: they say, the queen gave birth to someone unknown, "neither a son nor a daughter." Saltan is upset, but orders to wait until his return. The sisters substitute the letter, and as a result, the girl and her son are sealed in a wooden barrel and thrown into the ocean in the best sadistic traditions.

1960. There's an outbreak of smallpox in Moscow. This incurable deadly disease was considered conquered in the USSR. To fight the possible epidemic the KGB assigns colonel Shilov. He dives into the struggle at once because he has an old account to settle with smallpox. The task is to locate all those infected and stop the spread of the virus. But it is not an easy mission. The infection has spread to the underworld of the capital. Shilov must look for answers in the depths of the criminal world, unearthing its inhabitants and secrets.