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The construction of the Kremenchuk hydroelectric power plant is in full swing, but the Dnieper may flood the construction site. In order to save people, the chief engineer Ilchenko decides to blow up the newly built dam... The workers do their best not to let this prevent them from delivering the HPP on schedule.

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.