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!

When his wife Emily Kempson dies of a heart attack, it frees Henry Maurier to marry his young mistress. Confiding her suspicions that Henry murdered her patient, Emily's nurse is encouraged by a neighbor to tell the police. Henry finds himself wrongly convicted and awaitng his fate while the family doctor races against time to find the real killer and save his life.