In 1845, a Reading bookseller named John Snare came across the dirt-blackened portrait of a prince at a country house auction. Suspecting that it might be a long-lost Velázquez, he bought the picture and set out to discover its strange history. When Laura Cumming stumbled on a startling trial involving John Snare, it sent her on a search of her own. At first she was pursuing the picture, and the life and work of the elusive painter, but then she found herself following the bookseller’s fortunes too – from London to Edinburgh to nineteenth-century New York, from fame to ruin and exile.