In one of the southern ports, state security authorities arrested a spy with a plan of the object of sabotage - a floating dock prepared for towing to a remote port. Major Ludov guesses by a number of signs that an entire group of enemies is operating in the city. With the help of Rakitina, the librarian of the floating dock, Ludov and his deputy Savelyev manage to unravel the entanglement of espionage plans and prevent crime.
