El Doctor, allegedly archaeologist, has hired Didi and his boat to have someone drive him to a lake, where the "Santa Pioca", a Spanish galleon, has gone a long time ago. He forgets to tell that a treasure was located on board the ship. When Leo along with Marsupilami, which shows up as amphibian floats to the wreck, suddenly chief Yam appears and warns all parties against a curse that lies on the treasure.
Lors d'une baignade, Théo et Marsu découvrent une épée ornée de mystérieuses armoiries. Jean-Pierre fait expertiser l'arme par deux archéologues. Surprise : elle faisait partie d'un trésor oublié qui se trouvait jadis à bord d'un galion espagnol...
