Marie-P made a mysterious discovery in the pocket of a jacket that belonged to her grandfather: an old key that appears to have been used extensively. But what was it for? Our budding detective immediately begins to work. Marie-P search the house, in vain. She questions his family, with no results. Somewhat discouraged, she consoles herself by going ... stuff yourself with chocolate cookies triple Madame Lucia! Marie-P she found the key to the puzzle?