The day that he met Muhammad Ali, Malcolm X had no idea who he was. The ''greatest'' had yet to prove himself as he hadn't yet beaten Sonny Liston to become world heavyweight boxing champion. It was 1962 and that title match was three years away.
The black Muslim minister and human rights activist was surrounded by followers at a diner in Detroit when the handsome sports star walked in, but the significance of Ali's interest in Islam didn't register. As is said in this rich and informative doco, they were worlds apart.
However, Malcolm X did notice that young Cassius Clay, as he then was, had outstanding charisma. How could you not? The priest of the Nation of Islam knew little of sport, but there was something contagiously engaging about the younger man. It impressed him greatly.
In the turbulent years that led to the assassination of Malcolm X in 1965, the friendship that developed between them only lasted a short while. When Malcolm X was expelled from the Muslim brotherhood prior to his death, Muhammad Ali had stuck with the movement's leader, his mentor Elijah Muhammad.
If Muhammad Ali was looking for a role model in deeply racist 1960s America, he was sticking to the safer option. For the time being, at least.
Ultimately, as Ilyasah Shabazz suggests, knowing her father Malcolm X gave Ali the power to speak up. And speak up he did, passing comment on subjects from the Vietnam War to Jim Crow inequality.
Muhammad Ali said that he regretted falling out with Malcolm X. Though he was not always the most reliable witness to personal history, this prompts us to ask what might have been. If their friendship had been long lasting, it could have had enormous influence on black men and women in the US, and beyond.
If the relationship between these two icons of black American culture had prevailed, the FBI and various security interests would have taken an interest too.
The brief years of friendship are the focus of this documentary feature, directed by Marcus A. Clarke. It is based on a book by Randy Roberts and Johnny Smith titled The Fatal Friendship between Muhammad Ali and Malcolm X. Both authors are among the interviewees.
The nature of this friendship is somewhat inconclusive, a space where there seems to be not quite enough evidence and a wealth of opinion. Perhaps there was some subterfuge. Ali's career was directly threatened by his religious and political beliefs.
Among interviewees are family members and various associates. Muhammad Ali's younger brother, Rahman, and his business manager, Gene Kilroy, reminisce, while academics Todd Boyd and Cornel West provide wider context. And there is archival footage of various key figures, like Elijah Muhammad.
This is fascinating overall, even if the quality of some of the views is uneven.
Of course, the filmmakers would never have found themselves short of archival material with a subject like Muhammad Ali. He was such a magnet for the media until he so sadly succumbed to Parkinson's disease in his later years.
To have been friends for only a short time when the civil rights movement in America was building a critical momentum, seems like a lost golden opportunity.
But Malcolm X alienated himself from mainstream America with inflammatory views, that still shock today.
He remains a shadowy presence, despite the fact that Denzel Washington portrayed him in Spike Lee's 1992 popular biopic Malcolm X.
That Muhammad Ali and Malcolm X were friends, even for a short time, is intriguing. Far more interesting is the way the film follows the political evolution of the younger man, Muhammad Ali, in search of a mentor and a role model.
As Professor Boyd from the University of Southern California says, the two men met at a moment of transition.
It was a point in history when the perspective of black people on themselves and their history, and of the world in general on its peoples of colour was undergoing a mighty change.
Blood Brothers: Malcolm X and Muhammad Ali tells an engrossing story of untapped potential. Yet it is also another perspective on the sports superstar who was one of the most celebrated and significant figures of last century.