"This is a portrait of Diego Velázquez, from his arrival at the court of King Philip IV of Spain in May 1622, to his death 38 years and several hundred paintings later. It is a portrait of a relationship that is not quite a friendship, between a king and his subject and between an artist and his subject. It is a portrait of a ruler, always on duty, and increasingly burdened by a life of public expectation and repeated private grief. And it is a portrait...

When John Snare, a nineteenth-century provincial bookseller, traveled to a liquidation auction, he stumbled on a vivid portrait of King Charles I that defied any explanation. The Charles of the painting was young, too young to be king, and yet also too young to be painted by the Flemish painter to which the work was attributed. Snare had found something incredible; but what? His research brought him to Diego Velazquez, whose long-lost portrait of...