Frank Langella and Christopher Plummer

He could float like a butterfly and sting like a bee. That we remember. But a lot of us have forgotten — and most younger people haven't a clue — that the world's most famous prizefighter, Muhammad Ali, lost many of his prime boxing years ­because he refused to take part in the Vietnam War. The HBO movie Muham­mad Ali's Greatest Fight reexamines his politically charged battle for a conscientious-objector deferment — which Ali took all the way to the United States Supreme Court — with a starry cast that includes Oscar winner Christopher Plummer, Frank Langella and Danny Glover. But no one is playing Ali.

"Nobody should!" says the film's director, Stephen Frears, who has ­instead included dazzling archival footage of Ali waxing eloquently — and off the cuff — when he refused, for ­religious reasons, to be drafted into the military. "Ali was so fantastic, so beautifully spoken, so profound and artful when he met with the press that no writer could improve upon it," says Frears, who guided Helen Mirren to an Oscar in The Queen. "And really, why cast an actor when Ali tells the story so well himself?"

The film is set in 1971, a few years after Ali — who had joined the Nation of Islam and changed his name from Cassius Clay — was convicted of draft dodging, stripped of his world heavyweight title and banned from boxing. The Supreme Court case to appeal that conviction, known as Clay v. United States, went before nine mostly conservative old men, led by the Richard Nixon-appointed Chief Justice Warren E. Burger (Langella).

"It didn't look at all good for Ali," notes screenwriter Shawn Slovo. "But eventually, Justice John Harlan" — played by Plummer — "had his eyes opened by his young law clerks, who had their fingers on the pulse of the country." Harlan flipped the other justices in Ali's favor, allowing him to go free. "That was especially extra­ordinary because Harlan was dying of spinal cancer," Slovo says. "He so easily could have said 'F--- it' but didn't."

Plummer was originally asked to play Burger but instead went after Harlan. "I'm usually cast as the glib, rather cold fish — why do it again?" says the actor, 83, with a laugh. "I so wanted to play this man of great warmth and humanity, a man who was toughly conservative yet intelligent and open enough to have doubt. Can you imagine that? Doubt! Couldn't we all use a little more of that today?"

Muhammad Ali's Greatest Fight airs Saturday, Oct. 5 at 8/7c on HBO.

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