The yardstick no longer exists for us to measure how important Joe Louis was between 1935 and 1950. Sports today provide marginal TV entertainment that viewers can take or leave — no sport more so than boxing, which has one spectacular pay-per-view event every year or so and otherwise is relegated to late-night cable time slots. Joe Louis’s big fights were events that black Americans, then white Americans and, finally, people all over the world gathered around their radios to experience.

The distance between then and now will be illustrated Saturday night on HBO when the new documentary “Joe Louis: America’s Hero … Betrayed” airs at 8 p.m., right before the heavyweight championship fight between Vladimir Klitschko and Sultan Ibragimov. No matter how good the fight is — no matter how good any fight today is — there’s little chance of it winding up in a documentary some 70 years from now. HBO’s new film revives Louis’s life and legend, but it’s a sad fact that his story needed to be revived at all. The son of an Alabama sharecropper, Joe Louis Barrow grew up in Detroit, skipped violin lessons to learn amateur boxing, and became the first great black heavyweight since Jack Johnson, two decades before him. The bare facts are amazing enough. Louis owned the title between 1937 and 1949, defending it 25 times despite years lost to military service — a record not even Muhammad Ali and Mike Tyson were able to approach.

Such an achievement would be eye-opening in any era, but Louis fought his way to the heavyweight title at a time when Major League Baseball and virtually every other big-time sport was barred to blacks; simply put, he was just too good to keep from the heavyweight title. As every American should know, Louis was beaten by former heavyweight champion Max Schmeling in 1936 in one of the biggest upsets in boxing history, recovered to regain the title the following year, and then, in the most publicized sporting event of the century, knocked out Schmeling in one round in a 1938 rematch.

Schmeling, an amiable apolitical German who let himself be used by Nazi propagandists, was a foil who couldn’t have been created by Hollywood. Nothing could have galvanized the country in the years before World War II like a black American fighting a symbol of Aryan supremacy, and the irony that Louis and Schmeling later became pals and hugged each other on television during a 1950 telecast of “This Is Your Life” only adds to the story’s incredibility.

Louis was, foremost, the first great role model for what would become, from World War II forward, an ascending black middle class — in his public life, anyway, which was carefully orchestrated. As it turned out, Louis carried on numerous affairs that would have scandalized whites and blacks alike, including one with the Norwegian skater Sonja Henie. As Congressman Charles Rangel — who is joined as one of the film’s dozen or so talking heads by Maya Angelou, Dick Gregory, Louis biographer Chris Mead, Gerald Early, and legendary sportswriter for the Daily Worker, Lester “Press Box Red” Rodney — puts it, “When I was a kid, Joe Louis was everything. He was the epitome of racial pride.”

For more than a decade, black America built its social schedule around Louis’s fights (dinner before, party after), and soon after the Schmeling fights, Louis became a sporting hero of a stature that could not have been envisioned before him. That Joe Louis isn’t a household name today is due to his being sandwiched in history between two “bad” black men: Jack Johnson, who enraged white America by taunting his opponents and flaunting his white women before World War I, and Mr. Ali, who publicly opposed the Vietnam War and was openly contemptuous of white America. Johnson was a hedonist and Mr. Ali a spokesman, and as HBO’s film makes clear, Joe Louis was neither. He was a relatively ordinary man of enormous skill who endured and prevailed under extraordinary circumstances. In Mr. Gregory’s words, “This was the first and only time in the history of America that a black man ended up being a white hope.”

When Louis’s story was told in documentary form in the 1960s and ’70s, the story always ended happily: Louis, after his amazing career, was seen shaking hands with Frank Sinatra and other celebrity friends in Las Vegas, or playing golf with Sammy Davis Jr. in Palm Springs. “America’s Hero … Betrayed” tells us what the other documentaries didn’t: that the golf courses in the late 1950s were segregated, even for former heavyweight champions who were symbols of resistance to Nazi Germany, and that Louis’s Las Vegas job as a casino greeter was humiliating.

If the inclusion of the word “Betrayed” in the title seems a tad hyperbolic, wait until you have experienced the sight of a 37-year-old Louis, his reflexes gone, pummeled through the ropes in 1951 by a young Rocky Marciano, or performing a fake fight in a wrestling match, all in a futile effort to pay his back taxes (“The government wanted their money,” he once said, “and I had to try to get it to them.”). Surely no great American sports hero was ever subjected to a more degrading post-career than Louis, who forfeited four years of his prime to military service and raised a lot of money for the war effort. (In a haunting twist of fate, Louis’s 92-year-old sister, Vunies High, was found frozen to death outside her apartment at an assisted-living center in Southfield, Mich., just this week.)

“Joe Louis: America’s Hero … Betrayed” is at once exhilarating and sobering, restoring Louis’s glory while presenting the ultimate cautionary tale about the fleeting nature of fame in America. If its point of view seems to come a bit from the outside, if it never truly succeeds in getting inside the head of its subject, it’s no wonder: From the earliest films of Louis as an amateur boxer to those made just before his death in 1981, he always seemed a bit bewildered by the enormity of the role he was asked to play.