‘The Kings’ takes us back to a time when Leonard, Duran, Hagler and Hearns ruled the world

Serious boxing fans know the names of Jermall Charlo and Errol Spence Jr. and Demetrius Andrade and Brian Castano, but with all due respect to their respective skills, nobody can make the argument these champions are household names who have transcended their sport to become mainstream celebrities. You don’t see them joshing with Jimmy Kimmel or doing breakfast cereal commercials or guest-starring as themselves in sitcoms or blowing up Instagram.

There was a time, though, when the medium weight class of fighters — junior welterweights and welterweights, junior middleweights and middleweights and super middleweights — was sparkling with superstars who were globally famous, who were lionized in their hometowns and home countries, who dominated the sport with their speed and their punching power and their wildly contrasting personalities. They graced the covers of magazines, they were mobbed by fans, they were given parades, they were on TV all the time — and when they fought, especially against one another, those fights were events.

Their names were Leonard, Duran, Hagler and Hearns, and they were kings.

Premiering Sunday at 7 p.m. on Showtime, the four-part documentary series “The Kings” is a comprehensive and thrilling time capsule chronicling the boxing renaissance in the late 1970s that continued through the 1980s, with Sugar Ray Leonard, Roberto “Hands of Stone” Duran, Thomas “Hit Man” Hearns and Marvelous Marvin Hagler captivating millions with their generational fighting prowess, their interlocking love-hate rivalries — and some of the most memorable battles the sport has ever seen. Each episode contains a treasure trove of archival footage of the boxers in and out of the ring, off-camera interviews with the four legendary fighters (including memories from Hagler, who died last March) as well as with journalists, boxing historians and analysts and some valuable historical and political context, whether we’re seeing the parallels between Leonard’s carefully cultivated public image and Ronald Reagan’s “Morning in America” campaign or hearing Duran talk in his typically blunt style about the complicated and contentious relationship between his homeland of Panama and the United States. (Spoiler alert: He grew up resenting the United States and Americans and carried that chip on his shoulder for decades.)

“The Kings” kicks off in 1976, as Leonard skyrocketed to fame by winning the gold medal at the Montreal Olympics and charming Howard Cosell and much of the country with his smooth style in the ring and his telegenic looks. By 1979, Leonard was a bona fide celebrity who was making seven figures per fight, and he became world welterweight champion in prime time when he defeated an aging but still dangerous Wilfred Benitez by TKO.

On the undercard that night: Vito Antuofermo vs. Hagler for the middleweight championship of the world. (The fight ended in a draw.) Hagler was paid about 1/10th of what Leonard made that night and received about 1/100th the attention — a pattern that would continue through the years, with Leonard soaking up the spotlight while Hagler tried, sometimes in painfully embarrassing fashion, to reach equal footing as a crossover celebrity. (Frustrated by the media refusing to call him “Marvelous,” Hagler legally changed his name to Marvelous Marvin Hagler so they’d HAVE to recognize his self-given nickname.)

We also pick up the stories of Hearns, the pride of Detroit at a time when Detroit didn’t have a lot about which to be proud, and Duran, who was a few years older than the other three fighters, spoke almost no English, reveled in his reputation as a savage in the ring and a hedonist between fights — and just might have been the greatest pure boxer of the group. All four of these fighters grew up in poverty, but they took very different paths to the top — and some handled their success better than others.

Mostly, though, “The Kings” is about the epic battles. The four kings had nine world-title fights between them, including the Leonard/Duran “Brawl in Montreal,” the infamous “No Mas” rematch between the two, the still-controversial split-decision victory by Leonard over Hagler and the Hagler/Hearns war in 1985 that featured arguably the most memorable first round in the history of boxing, regardless of weight class.

We may never see another boxing era like the reign of these kings.