A stranger knocks on the door of the apartment occupied by the R. family. He warns them that an epidemic is spreading in town. Death usually ensues in 3 days and is preceded by swelling, blisters, and redness of the skin. Mice are suspected to carry the disease. The young man appears ill but claims to be a survivor and now immune to the epidemic. He advises the family to remain indoors, avoid mice, and practice strict hygiene. He offers to bring food. The family is skeptical and declines his offer of assistance.

Soon the city is ravaged by the disease. TV and phone stop working. Violence and looting are rampant. Nikolai, the father, regularly goes out at night to rob food and supplies for his family. Sometimes he kills. When he returns home, he always cleans himself thoroughly. He lives with his wife, Elena, their daughter, and Elena's parents.

The family's cat is outside on the balcony and hungry so they bring it inside. The animal eats a mouse, and afterwards the little girl kisses the cat on its mouth. The adults are horrified. They quarantine the child in her bedroom along with the cat. After 3 days, there is no sound or activity in the bedroom. The girl is presumed dead. The cat is alive and escapes. The child's parents and grandparents manifest signs of the infection and die.

Six days after his initial visit, the young man who warned the R. family returns to the apartment building. The place is silent except for the meowing of a cat. The stranger breaks into the apartment and sees 4 dead bodies. Inside the barricaded bedroom, he finds the little girl alive and recovering from the infection. Next to her in bed is the pet cat.