I liked the Doctor Who version: Evil-Alien-Disguised-As-Human plans to make a fortune by using time-travel to commission Leonardo to paint many Mona-Lisas, which he will then sell in the future, making a fortune since they will all be real and each collector will be convinced he has the actual painting.
The Doctor foils the plot by taking a magic marker and writing "This Is A Fake" on all the blank canvasses Leonardo has set aside for the project.
