In one of the southern ports, state security authorities detained a spy with a plan for the object of sabotage — a floating dock, prepared for towing to a remote port. Major Lyudov, according to a number of signs, realizes that a whole group of enemies is operating in the city. With the help of Rakitina, the librarian of the floating dock, Lyudov and his assistant state security lieutenant Savelyev, they manage to unravel the tangle of espionage ties and prevent the crime.