Nelson Mandela died in December 2013 but he had long before been canonised as a secular saint. Many people – particularly on the political right – found it convenient to forget that for decades they had regarded him as a terrorist. He had become the world’s grandad: an icon of spiritual generosity and reconciliation.

This three-part series directed by James Rogan ends in 1994, when Mandela became president of South Africa and that process of sanctification was under way. It’s gripping, it’s revelatory and it pulls no punches. It evokes the grim reality faced by Mandela and his allies during their decades-long struggle against apartheid. It’s a world of white South Africans suggesting their Black compatriots had “only just come down from the trees”. Of British young Conservatives with their “Hang Nelson Mandela” posters. Of physical violence, emotional torment and awful economic unfairness.

It tells a sprawling story with many moving parts, both inside and outside South Africa. Mandela is mostly present as a looming absence – he is central to the narrative and yet, as a prisoner, aside from it. His half-life on Robben Island becomes a framing device, as an array of characters – lawyers and activists; journalists and judges; politicians and pop stars – are pulled into his slipstream. As Dali Tambo, the son of former African National Congress (ANC) leader Oliver, says: “He became more than himself.”

As Mandela and other ANC leaders languished in captivity, the series tracks the diasporic face of the resistance. Musicians Miriam Makeba and Hugh Masekela went into exile abroad but relentlessly spread the word. Via disrupted sporting events and street protests, the likes of Peter Hain engaged in activism in Britain. Inside South Africa, ANC fighters, such as James Mange were at the sharp end. Mange eventually ended up on Robben Island, where Mandela was startled by the movement’s new militancy.

But events recorded in history books are barely half the story. This telling brutally emphasises the personal cost of Mandela’s resistance. He lost his mother and son in consecutive years and mourned them remotely. His wife Winnie, meanwhile, is a complex figure about whom the series is wisely nonjudgmental. If, eventually, her radicalism became incompatible with her husband’s gentle pragmatism, it is made clear that she had every justification for her rage. Over the years, we see her harden, and no wonder. She was physically and mentally tormented by the South African authorities. She was forcibly moved to a town full of racist white Afrikaners. Her house was burned down. “My grandfather was insulated by prison,” says Nelson’s granddaughter Ndileka Mandela. “She was in the eye of the storm.”

The case of Winnie Mandela becomes a metaphor for the wider dilemmas Nelson Mandela faced. There’s an enlightening explanation of the philosophy of Ubuntu, which is rooted in various African tribes. It translates as “I am because you are”. It expresses intertwined humanity and is anathema to apartheid. But, as violence escalated through the 80s, it arguably wasn’t compatible with the anti-apartheid movement’s wilder fringes either. This was the treacherous ground Nelson Mandela was forced to navigate, and his three-way release negotiation – with the ANC, his fellow prisoners and the South African authorities – remains a miracle of diplomacy.

It was helped, however, by a world waking up to South Africa. There will be those who catch the faint scent of white saviour syndrome in the prominence given to the artists involved in the series of huge concerts first demanding then celebrating Mandela’s release. However, it’s hard to argue that mainstream British and American engagement with the resistance wasn’t a significant driver in apartheid’s demise. It’s also surely impossible to dispute that Free Nelson Mandela by the Specials is both the most joyous and most effective protest song of all time. Writer Jerry Dammers describes its performance at the 1986 Festival for Freedom as “the proudest moment of my life”.

By this time, it’s clear that the movement had gathered unstoppable momentum. The reminder of the BBC’s decision to screen Mandela’s 70th birthday concert in 1988 in the face of legal threats from Conservative MPs is poignant in the light of the BBC’s apparent unwillingness to interrogate hard right, anti-immigrant tropes. As Neil Kinnock puts it: “In the Commonwealth, South Africa’s only friend and defender was Margaret Thatcher.”

Given the flux of the world in 1990, Mandela’s release seemed inevitable at the time. This series shows that it wasn’t, and demonstrates how much could have gone wrong. The conclusion alludes to the Truth and Reconciliation process that took place in South Africa through the 90s but steps away at this point – Mandela’s post-prison life is worthy of another series in itself. Instead, alongside victory, there’s a lingering sense of loss.

Free Nelson Mandela is nuanced enough to explore activism as a life’s work: a road without end. Activist Barbara Masekela says of Mandela: “When you worked closely with him, there was always a kind of deep sadness.” Finally, this documentary suggests that Mandela’s impending sainthood was a product of that. His years of resistance sharpened him but tempered him too. By the time of his death, he had come to represent light. But that was because he had spent so much time in the darkness.