The yakuza, Japan's homegrown mobsters, are favorites of local filmmakers but not documentarians, for reasons entirely understandable. A documentary that seeks to delve into the inner workings of the Yamaguchi-gumi might find an audience, but the hurdles to making it, such as scouting subjects willing to dish openly (and possibly suicidally) on camera, would be formidable. Better to make another TV-friendly program on tuna fishermen.

Documentarian Ken'ichi Oguri, backed by uber-producers Kazuyoshi Okuyama and Chihiro Kameyama, has finessed this difficulty by focusing his new film, "Tokyo Joe: The Man Who Brought Down the Chicago Mob (Mafia o Utta Otoko)," on Ken Eto — a Japanese-American FBI informant who put 15 Chicago mobsters and mob associates behind bars in the 1980s.

Eto was no ordinary snitch. Born in California in 1919 and raised by a harshly disciplinarian father, Eto was a wild, scrappy and highly intelligent kid. He found his true metier in a World War II detention camp, where he fleeced fellow detainees in poker games. After the war, he settled in Chicago, where he honed his skills in card sharping while insinuating himself into the mob-run gambling business.