The name Vivian Maier was all but lost to history until 2007, when John Maloof, a flea market merchant in Chicago, stumbled upon some 120,000 negatives that revealed the never-before-seen photographs of this nanny–turned–American street photographer, who died in 2009. Maloof showed many of them in an exhibition at the Chicago Cultural Center in 2011, and overnight the reclusive woman became recognized as perhaps one of the most intriguing outsider artists. Now comes Finding Vivian Maier (out March 28), the absorbing documentary Maloof made of his quest to find out just who the inscrutable Maier—a prolific artist who kept her archive hidden—really was. “She had great sense of framing,” the photographer Mary Ellen Mark tells Maloof as she looks at Maier’s images of construction workers and invalids. “She had a sense of humor, a sense of tragedy.” While her former employers (among them Phil Donahue) offer insights into Maier’s personality, Maloof sorts through the ethics of exposing her tightly guarded work. She was self-taught, he notes. “She just knew what she was doing.” And now, so do we.