During Stalin’s rule of the Soviet Union in the early 1950s, Ministry of State Security (MGB) Agent and war hero Leo Demidov uncovers a strange and brutal series of child murders. However, MGB leadership refuses to acknowledge the deaths as murders because Soviet doctrine states that only capitalism creates serial killers. The son of Leo’s partner is murdered by the serial killer and during Leo’s investigation, his wife, Raisa, is accused of being disloyal to the state. Leo suspects that his amoral and ambitious enemy in the MGB, Vasili Nikitin, is behind the charges. Leo refuses to support the accusation and he is forced to take a demeaning militia position in the town of Volsk. Raisa accompanies him and must work as a janitor.
In Volsk, Leo meets colonel Mikhail Nesterov, his new commander. Meanwhile, Vasili calls Raisa and attempts to persuade her to leave Leo and join him in Moscow. When she refuses, Vasili orders a local MGB functionary to abuse her. Raisa later admits to Leo that she only agreed to marry him because she feared what would happen to her if she refused a high-ranking member of state security.
More child murder victims are discovered in Volsk and after Leo tells Raisa that he suspects this to be the work of a serial killer, she decides to help his investigation. Together, they convince Nesterov and his wife, Inessa, that the deaths must be investigated as serial murders. Further investigation reveals that the killer has claimed at least 44 victims and that he is traveling the rail lines to find his targets.
Leo and Raisa travel in secret to Moscow to interview a woman who reported seeing the killer. The interview does not bear fruit, however, and to escape possible pursuit by the MGB, they seek out Raisa’s former colleague. However, during the meeting, Raisa discovers that the colleague is an informer and Leo kills him. Leo tells Raisa that she can leave him if she wishes but she says that she wants to stay with him.
Leo and Raisa return to Volsk where they discover that Vasili and his men have pursued them for killing the informer. They are arrested and interrogated, then put on a train to a work camp. During the train ride, they are attacked by the other passengers on orders of Vasili; after killing their assailants, Leo and Raisa jump off the train. They trek to Rostov where the highest concentration of the serial killer’s victims has been found; they reason that the killer must work close to the rail yards there.
In the Rostov tractor factory, Leo identifies Vladimir Malevich as the killer by cross-referencing worker travel with the location and date of the murders. Leo and Raisa corner Malevich who surrenders to them. However, Malevich is suddenly shot in the head by Vassili, who has followed Leo and Raisa. Vassili tries to execute them, but after a vicious struggle, they kill him. Leo cleverly tells the MGB agents who arrive that Malevich killed Vassili and that he then shot the killer. Because Malevich was a POW in German camps, the MGB is able to (falsely) explain away his actions as those of a Nazi agent.
Leo is reinstated in Moscow; despite being offered a promising political position, he instead asks to create and lead a homicide division in Moscow. Leo and Raisa adopt two orphan girls.
