When Geronimo Stilton finds out that the Pirate Cats are going back in time to Venice in the year 1517, he knows they must be after one thing: The Mona Lisa! Geronimo and friends must travel to Renaissance Italy to stop the Pirate Cats from stealing the Mona Lisa and taking credit for what is arguably the world¡¯s most famous painting. But the Pirate Cats have an ace up their sleeve: someone in Geronimo¡¯s crew is an imposter!