When Ethan and Ella first land in Venice, they are in awe of all the canals and tiny little streets. They're especially excited to look for a five-hundred-year-old-mosaic near Calle Farnese that Grandpa Harry tells them about. While the twins are out getting gelato later that day, they notice someone looking around suspiciously as he unties a gondola from a dock. The next morning, the twins hear two gondoliers arguing, one of them accusing the other...