Lady With an Alien

A time traveler accidently leaves his pet behind in the Renaissance, where Leonardo da Vinci takes it in. The teenaged owner wants the pet back, Leonardo wants to know about the future. They make some uneasy bargains, Leonardo shows the boy around -his- Milan (which is not on the tourist circuit), and the boy is there when Leonardo paints his famous "Lady With an Ermine", which of course isn't an ermine at all.