Crooks, Freddy, Max and Theo rob a payroll lorry in the desert. Poly witnesses the crime taking place but the raiders are not concerned as they believe that a Pony cannot incriminate them.
Poly is smarter than your average pony and leads the mine workers to the stranded payroll lorry. Victor finds a medal belonging to Mustapha's father in the sand. Freddy left it there to deflect blame onto the local Tuaregs.
Theo finds the medal hidden by the children and hands it over to Inspector Ben Arfa accidentally incriminating himself in the process. Kerim heads off into the desert hoping to find Mustapha's father before the police.
Freddy and Max masquerade as street merchants in the town of Tozeur while they wait for the investigation trail to go cold. Poly recognises them as the robbers and tries to raise the alarm.
Freddy and Max are delighted when the police inspector arrests Mustapha's father for not having an adequate alibi. Kerim sets out alone to find a witness to back up Mr. Daouri's story.
The boys set off into the desert after Kerim. By chance they meet Mr. Fleur on one of his botanical field trips. He seems to be the very witness they have been hoping for. The boys are delighted but Freddy and Max most certainly are not.
Freddy and Max decide to kidnap Mr.Fleur so that he is unable to testify against them. They leave him stranded in the desert but Poly soon rescues him. Later, Max flees after Mr.Fleur recognises his voice and raises the alarm.
Freddy decides to flee alone with the stolen wages but is thwarted once again by Poly and the children who recover the money. Determined to continue his crooked ways he steals a unique and important book from Mr. Fleur's office.
Only Mr. Fleur can decipher some valuable formulas contained in the book, so Freddy and his new accomplice 'La Fouine' put a kidnap plan into operation involving a helicopter.
Mr. Fleur is taken to the crooks secret hideout. They threaten to harm Kerim and the children if Mr. Fleur refuses to cooperate. Reluctantly he has no choice but to obey their demands.
The children team up with Kerim's antique dealer friend, Azis, to rescue Mr. Fleur. The attempt fails and Azis is arrested. Meanwhile, La Fouine takes Mr. Fleur's book to show to Baron Tosca.
The two boys and Poly find their own way to Baron Tosca's home and are surprised to find a girl there called Sonia who looks exactly like Nathalie. Mr. Fleur is also there, forced to work for the Baron.
Mr. Fleur relates the story of twins, Nathalie and Sonia to the boys. Poly takes a message to the police who arrive at Baron Tosca's to arrest Freddy and La Fouine. Jean Forestier arrives also, hoping to be reunited with his two daughters.