Seryozha's father - engineer Batashov - is arrested for losing a secret document. The guilty mother-in-law marries another and leaves for a seaside resort.
Their apartment is transformed into the residence of a spy posing as "Uncle Vasya". Become a blind weapon of the gang, Seryozha realizes at the crucial moment of what happened and helps the Chekists to stop the enemy ...
The film features the song "The drums beat" ("Hey, the inevitable combat detachment goes up a steep mountain towards the battle"), music: Lev Schwartz lyrics: Yakov Akim
