In 2007, a Chicago real-estate agent bought a box of undeveloped negatives at auction for four hundred dollars and uncovered one of the greatest unknown photographic archives of the twentieth century. The images belonged to Vivian Maier, a reclusive nanny who quietly documented street life in Chicago and New York for decades yet never showed her work to anyone. Her death in 2009—just before the world discovered her genius—makes her story one of the strangest in modern art.