The children, living in a small coastal town, notice old Balyu hanging around the derelict windmill on the shore. He pays his debt to the con man Morti in a gold piece. The boy Kolkata manages to hide the coin and shows it to the Professor, an art historian. He concludes that the coin is exceedingly valuable from a cultural and historical point of view. One night Morti and his assistant Moks wring the secret from the old man, take the gold and start for the border in a boat. The police speedboat overtakes the thieves. Old Balyu takes the children and the Professor to a secret vault where for years on end he has been guarding a large gold treasure.