Botticelli's Mystic Nativity is a painting that lovers of mystery fiction should love: a Renaissance masterpiece crammed with cryptic symbols disguising a dangerous message. But it is much more besides. Painted in 1500 by one of the most famous artists of all time, it is a supremely beautiful vision of maternal love, earthly harmony, and heavenly ecstasy. But the painting also has a dark side. It was inspired by the preaching of Savonarola, the puritanical friar who held Florence in his grip in the 1490s. He purged the city of non-Christian art, destroying it on the notorious Bonfire of the Vanities. But he himself met a violent, fiery end, and Botticelli had to carefully hide the Mystic Nativity's dangerous meaning. Only a recent chance discovery by a scholar fully unlocked the painting's message. The painting was the first Botticelli work to arrive in Britain, brought here in 1799 by a wealthy young owner of slave plantations. It would have cost him just a few pounds at a time when Botticelli's name was all but forgotten. But before long, the painting would be on display to an audience of millions at the world's biggest ever art exhibition held in industrial Manchester.

• Samuel West