"The Mona Lisa is simply the most important single painting in the world," says one pundit in The Missing Piece: The Truth About the Man Who Stole the Mona Lisa, director Joe Medeiros' documentary about Vincenzo Peruggia, an Italian thief who stole the painting and then held it hostage for two years. Medeiros admits to being obsessed with the story for 30 years, and he sets out to find out what Peruggia's real motive was. Light-hearted and funny, the film offers an intriguing look at bit of forgotten history. It screens tonight at 7 and at 1:30 p.m. on Sunday at the Cleveland Museum of Art. Tickets are $9. (Niesel)