In this adventure, Tintin goes to a jungle in San Theodoros, South America to find an Arumbaya Fetish which has been stolen from a Museum. Two other people, Ramon and Alonso are also after the Fetish because they know that a diamond is hidden in it. Tintin gets stuck in a revolution in San Theodoros but manages to escape from there.

Tintin finds a briefcase at the Park and goes to return it to its owner, professor Alembick, who is a sigillographer. The professor is invited to Syldavia to study the royal seals. On Tintin's way out Tintin overhears his name in a conversation. Tintin follows one of the men and goes to a Syldavian restaurant where he overhears some more conversation concerning Professor Alembick's invitation to the Syldavia. That night Tintin gets a strange phone call from a man who wants to meet him about some important issue. However, when Tintin opens the door, the man drops senseless to the ground. When he comes around, Tintin finds that he is sufferring from amnesia. Intrigued by these incidents, Tintin decides to become the Professor's secretary and accompanies him to Syldavia. On the plane, Tintin suspects that the Professor he is traveling with is an imposter but is unable to prove it. They have to change planes and the pilot of the second plane tries to kill Tintin by ejecting his seat from the plane, but Tintin is saved by falling into a cart filled with hay.