Tony becomes acquainted with Stella's friend, Dominique the fisherman. Scientist Antoine explains to Tony that a blank piece of paper hidden in the medallion may hold a secret message.
Mr. Bonetti senses that the medallion may be valuable and tries to persuade Madame Calvi to part with it. Using a technique learned from Antoine, Tony discovers the secret message written on the paper.
Mr. Bonetti and his sidekick Joe increase their efforts to obtain the medallion. The children are one step ahead though, and begin searching for the first of seven hidden stars.
Stella has no choice but to hand over the medallion to Joe. Poly leads the children to the first star. They are not too happy when Antoine calculates the location of the second star.
After an early miscalculation is corrected the children make progress searching for star number two. Bonetti threatens Madame Calvi after he discovers the secret paper is missing from the medallion.
The children worry that their treasure hunt may have come to an end when the latest paper clue turns out to be blank. Dominique earns extra money to help Madame Calvi avoid eviction.
The children get themselves organized in order to rescue Poly. Later, Joe overhears Antoine explaining to Tony and Dominique how to find the next star. He rushes to inform Bonetti.
Joe arranges for his gang to ambush Tony and Stella but Dominique intervenes. Mario Bonetti decides that it is finally time to use ruthless measures to get what he wants and sets out to threaten Madame Calvi.