John Maloof is around 30, and has spent much of his life going to garage sales and buying up the contents of storage lockers. In 2009(?) he bought a storage locker lot that included a large quantity of photographic negatives, hoping for images of Chicago for a project he was working on. That didn't work out, but what he got instead were some brilliant photos: photographic negatives are the kind of thing you usually throw out when you find them in the storage locker, but not these ones. So Maloof went and bought up the other lots from the same locker, and began inventorying everything. The photographer in question was Vivian Maier. Maloof convinced one gallery to show her work - I doubt he needed to do any convincing after that, because her posthumous rise to fame has been explosive. And with good reason: she was a brilliant photographer.
Much of the film follows Maloof as he interviews various people who knew her, and even ends up taking an excursion to a small town in the French Alps where she had travelled and had some cousins. She was employed as a nanny - photography was something she did all the time, but she never showed her images to anyone during her lifetime. Highly recommended.
And finally, a photo I blatantly lifted from that site - I chose one I liked that hasn't been getting as much press as some of the others: