When Mystery Inc. visited their friend Antonio in Venice, they ran afoul of the Ghostly Gondolier in his medallion stealing scheme. When Daphne Blake tried on the 3rd medallion at the museum, the Gondolier simply kidnapped her to get the medallion and used his deflatable rubber gondola to escape. He then kidnapped Antonio to get the 4th and final medallion.
The Gondolier attacked the gang one last time. This, however, led Scooby-Doo to discover the ghost's hideout. After the prisoners were released, Velma Dinkley realized the Gondolier would be going to the Plaza de san Marcos, as depicted in the picture. They caught up with him as he found the treasure, and he was captured at the end of a long chase. The Ghostly Gondolier was unmasked to reveal Mario.