Varya Ivanitskaya has a roof over her head, a favorite job at the Institute of Biological Problems, a friend Ida, who is trying to force her to live by her own rules, and her beloved dog, the fox terrier Persik. One day, Varya accidentally overhears a conversation: some man is threatening Ida with death. . The girl is sure that this is the Don Juan and scoundrel Slivkov, who recently abandoned Ida. But the next day Slivkov unexpectedly dies himself! And the city is overwhelmed by a series of terrible deaths - the victims die with a terrible grimace on their faces, and a certain mysterious woman is always next to them. Varya decides to find out what is killing the unfortunate people and who this fatal stranger is...