Seven years ago, a young Chicago historian named John Maloof made an extraordinary discovery. He picked up a box of undeveloped photo negatives at an auction belonging to a mysterious woman named Vivian Maier; later, Maloof tracked down a storage unit rented in her name, filled to the brim with negatives, prints and miscellaneous effects.

For a modest payment, he found himself the owner of a staggering, huge archive of street photography by a brilliant, undiscovered talent, clearly to be compared with Henri Cartier-Bresson and Diane Arbus. These were thousands of stunning images taken on the streets of New York and Chicago from the 1950s to the present, but never shown to anyone in the photographer's lifetime. This documentary shows Maloof's mission to develop, catalogue and publish this sensational trove, and to find out more about the unknown artist herself. Maier, who died in 2009, had earned a crust as a nanny to the well-heeled, dragging her charges out on long walks while she took candid shots on the streets, and also dabbled in Zapruder-ish cine film. Her humble job allowed her to roam, and perhaps her low status gave her a sharp sense of dispossession and even resentment. Interestingly, the pictures she took in rural France, her mother's birthplace, are calmer and gentler than the fierce images of Chicago. These are, in a sense, symptoms of her own mental turmoil. This is a fascinating study.