Abundant supplies of imported vaccine prove ineffective against an outbreak of foot-and-mouth disease epidemic in several population centers. Warehouse manager Rashkov has been found dead. Major Zahov has to investigate the case. Rashkov has been poisoned. The "unidentified person" has poured poison into his water glass, has taken a document identifying the real saboteur and has vanished. The "unidentified person" turns out to be Irina, a young and beautiful assistant employed in the office. She has been blackmailed into becoming a reluctant murderer. Torn between his professional duty and his emotional involvement with the girl, Zahov experiences a personal drama, but he unravels the whole truth about the crime. At midnight, when he arrests the arch saboteur, Zahov loses Irina who commits suicide.