Anthony Mann's first big success is a tightly written pseudocumentary on undercover Treasury agents, which owes much to John Alton's stunning chiaroscuro camera work. Like Dragnet, which it resembles in the use of the voice-over narration typical of the period, the film was based on an actual Treasury Department case. In his Notes on Film Noir, director Paul Schrader wrote to the effect that noir is not a genre per se, with specific attributes of character and setting, but a style, based on tone and atmosphere. The film is nothing if not a triumph of style over substance, and while the byzantine plot is kept in sharp focus, it's the visual inventiveness of Mann and Alton that's makes it work. Film historian Todd McCarthy included scenes from the film in his acclaimed documentary on the history of cinematography, Visions of Light (1993).