Tyson laid bare
Although 25 years have since passed, Big Joe Egan still recoils as he recalls his first punch from Mike Tyson.
The Irishman had just been recruited as a sparring partner for the young star, who was already being touted as a future world heavyweight champion.
At first, Egan couldn't understand what all the hype was about.
"When I saw Mike, I was sure I could beat him," Egan told me. "We were both 17 and he was shorter and had less reach than me."
A few seconds in the ring soon changed his opinion.
"I will never forget that first punch he hit me with, a straight right to the body that literally lifted me off the floor," he winces, before adding, "he didn't put me down though."
In fact Egan was not knocked to the canvas in two years as Tyson's sparring partner, leading the American to dub his friend "the toughest white guy on the planet".
The pair became good friends, together building a coop for Tyson's beloved racing pigeons and watching videos of classic fights each evening.
They still speak on the phone and Tyson wrote the foreword to Egan's autobiography.
The film has transformed many people's opinions of Iron Mike.
Director James Toback admits: "I've been astonished by the reaction. Given the incendiary nature and reputation of the subject I thought I'd be lucky to get even a partially positive audience response.
"But even people who have gone in with a real hatred of Mike have come out moved and emotionally stirred."
The Tyson we see in the 90-minute documentary is vulnerable and driven by fear.
He talks about being "bullied and humiliated" as a fat, bespectacled child on the streets of Brownsville, New York, where he was abandoned by his father and left to fend for himself by his mother.
One of his starkest childhood memories is of a stranger throwing his glasses on the back of a milk cart for no reason.
Tyson says his aggression and assertiveness is the result of this bullying.
"I am just afraid of being treated that way again, of being physically victimised on the streets again. I was just afraid, so afraid."
Egan, who went the distance with Lennox Lewis and Bruce Sheldon as an amateur before his pro career was cut short by a car crash, says the film accurately portrays the man he knows so well.
"I have had long conversations with Mike about his problems," he says.
"He didn't have the fortunate upbringing most of us have. He was left on the streets as a child and had to fend for himself in Brownsville, one of the toughest environments you could possibly imagine. The film shows how vulnerable Mike is."
Tyson also reveals, for the first time, the breathing problems that have hampered him throughout his life.
This is one reason why he was desperate to knock his opponents out as quickly as possible and worried if he couldn't.
Toback says: "From the first few seconds of the first shot I was aware of it. When I put the headphones on I was shocked to hear how heavy and laboured his breathing was.
"I found it quite fascinating because he had never spoken about it anywhere else, probably with some justification, because it wasn't something he would have wanted his opponents to know."
It would be wrong to portray Tyson as a victim though, as he himself is only too willing to admit.
After all, he has done much to perpetuate the negative image that many have of him.
In 1992 he was convicted of raping the beauty queen Desiree Washington, which he still denies; he twice bit chunks from the ear of Evander Holyfield in their 1997 rematch and declared before his fight against Lennox Lewis in June 2002 that "I want his heart, I want to eat his children".
He has had persistent problems with drink and drugs and was surrounded by a coterie of hangers on throughout his career.
Yet the film reminds us why Tyson was one of the most enthralling sportsmen in the history of sport.
You can still sense the excitement of the crowd as he is shown advancing on the ring in his first world title fight, dressed only in black shorts and without a robe, exuding menace.
He climbs through the ropes before fixing his opponent, Trevor Berbick, with an unblinking stare.
His face is expressionless until the bell sends him surging forward, head down, to pursue his opponent with a flurry of punches.
With just 30 seconds of the second round remaining, he fells the WBC champion with a right to the body followed by a savage uppercut.
Tyson reveals he was fuelled by exceptional anger that night because of the discomfort of gonorrhoea and a sense of grievance at the beating Berbick had dealt to his hero, Muhammad Ali, five years earlier.
Tyson went on to become undisputed champion and many of his opponents seemed to be beaten before the opening bell had sounded.
"I was beating guys through pure brute strength and intimidation," Tyson admits in the film.
This aura of invincibility went when a Tyson diminished by drinking and partying was beaten by Buster Douglas in Tokyo in 1990 in one of the biggest shocks in boxing history.
Although he did eventually reclaim his undisputed crown, Tyson was never quite the same fighter again.
Egan believes the fighter, who is now 42, would have become the greatest boxer of all time were it not for the death of his mentor, Cus D'Amato, in 1985.
The legendary trainer had taken Tyson from a juvenile centre to live with him at his home in Catskill, New York.
Egan says: "If Cus had lived longer, I sincerely believe Mike could still be ruling the world.
"When Cus died, Mike lost his focus on boxing and his love for the sport. We have never really seen the best of him, despite what he's achieved."
Although he is composed for the rest of the film, Tyson breaks down in tears when he talks about his former mentor.
He had initially been suspicious of the old man's motives, but soon flourished thanks to the unconditional love and belief shown in him.
There is moving archive footage of an interview the duo did together in the mid-80s, when D'Amato says "To me, he's my boy, he's with me. I often say to him 'I owe you a lot' and he doesn't understand what I mean. Well I'm going to tell him now - if he weren't here now, I probably wouldn't be alive today."
It's sad to watch footage of Tyson's final fight, against journeyman Kevin McBride in June 2005.
When Tyson is pushed to the floor in the sixth, he doesn't seem to have the energy or even the inclination to get back on his feet.
When the bell signals the end of the round, he retires on his stool. The contrast with the ferocious, electric fighter we had seen in the 1980s could hardly be greater, yet Tyson actually seems relieved.
"I haven't got the fighting guts any more, I just don't have this in my heart any more," he says in his after-fight interview.
"I don't have that ferocity; I'm not an animal any more."
The 42-year-old's remaining ambition might appear modest, yet for him it has often seemed unattainable.
At the end of the film he says: "I just want to be a decent human being, which I know I can be."