Geronimo and the gang hear a rumor that there is another painting hidden beneath the "famouse" painting, the Mona Mousa. Of course, they can't resist the call of a mystery. They travel immediately to the mouseum where Thea sweet-talks the art expert into loaning her the CD of the eleven mysterious objects in the hidden picture. Cracking the code leads to a trip to eleven spots to find a letter in each. Putting the letters together properly will solve the mystery.

Alas! At the very first place, a fish market, poor Geronimo is splashed with water, slips on a fish bone, has a giant fish land in his arms, and gets a big slimy hug from an octopus! He and Theo eventually find the Pelican's Pillar, where they discover the first letter --- along with the news that an elderly lady was also looking for the Pillar. One by one, Geronimo and his sister find the places and letters, only to hear that other mysterious persons were there before them. Can Geronimo solve the mystery first?