Sathnam Sanghera’s mum doesn’t know he’s going to India. He tells her he’s going to Las Vegas. She’d only worry, he says. This was one of many lovely personal touches in The Massacre that Shook the Empire (Saturday, Channel 4), about the slaying of hundreds of Indians by the British-Indian army in Amritsar in 1919. In less than an hour, the programme succeeded in bringing together sensitive reporting of the massacre and the context in which it took place; angrily examining our “wilful amnesia” about the brutality of the British empire and our idea of British exceptionalism; and offering many insights into the second-generation experience.

Despite the grave subject matter, there was a lightness of touch, thanks to Sanghera’s charm as presenter. The weird nuggets of personal information he offers up – that one of his uncles killed another uncle, that he got a brain parasite the last time he visited India – bring a quirky edge. He speaks bad Punjabi to a group of men in the street, trying to find out where to buy a lathi – a long stick the British claimed was being used as a weapon by Indian protesters – and they think he’s after some lassi, “a kind of Indian yoghurt,” he deadpans.

But this is a serious documentary. A century ago, soldiers shot 379 (officially, though likely to be many more) people, including children and babies, who were at the Jallianwala Bagh park in Amritsar.

“I believe,” says Sanghera, “the Amritsar massacre was the moment Britain lost its empire.”

Churchill said later that it was an isolated event in British rule – the fault, essentially, of one rogue senior officer. He was Reginald Dyer, and Sanghera visits his great-granddaughter, Caroline – she has the look of an aged royal’s lady-in-waiting – to find out if his family live in eternal shame. But Caroline reveres him. He was an “honest, obedient soldier” who, she claims, was being charged at by 25,000 people, and is iffy about the murdered babies. I’m not convinced of the value of asking Dyer’s great-granddaughter what he might have been thinking, but their meeting is deliciously awkward. Dyer “had a lot of friends who were Indians,” she says. “We all adored the Indians and I’m delighted to meet you,” she tells Sanghera. “I think I’m probably more British than Indian,” he replies.

The unpicking of identity and the effect of the massacre down the century is the sparkling undercurrent here. Sanghera, whose parents emigrated to the UK from just outside Amritsar, looks uncomfortable on one of the Punjabi city’s narrow streets as a man drives up close behind him on a scooter. “If my parents had stayed here, this is the kind of place I would have grown up.”

Indian school students and teachers pay tribute a day before the 100th anniversary of the Jallianwala Bagh massacre. Photograph: Narinder Nanu/AFP/Getty Images

The site of the massacre has topiary in the shape of riflemen and white squares are painted on the walls to frame the bullet scars. A historian says he believes the true figure of those killed could be up to 600 – including bystanders enjoying the park with their families – and Sanghera reads some horrific testimonies of those who were there. As if the massacre wasn’t bad enough, in the aftermath the Brits implemented “fancy punishments”, which included public floggings, making Indians salute white people on the street (punished by torture if they failed) and a “crawling order”, where Indian men were made to crawl down a street. “It’s hard to read this,” he says of the testimonies, “and not feel physically sick.”

Sanghera – all slick metropolitan uniform of smart jacket and white trainers – is the product of his British education and says he hadn’t given much thought to what empire meant. Not many of us have, even though it explains so much about our inflated view of our position in the world – especially pertinent at the moment. “If we can’t be honest about Britain in India 100 years ago then how can we be honest about how we view Britain in the future?” asks Raj Kohli, a descendant of a survivor, at a meeting with Caroline Dyer – she is used in the most contrived ways, but at least is a catalyst for good quotes.

It feels like a genuine revelation to Sanghera that the British empire is, as he describes it, “an exercise in institutionalised racism”. Don’t many of us know this? Although it takes the specifics of a documentary such as this to confront it. In a taxi on the way to his family’s home in Wolverhampton he admits “it fundamentally changes the way I see my country and my place in it.” He wants the British government to apologise (it declined to do so to mark the centenary last week, although the prime minister, Theresa May, expressed “deep regret”). It’s hard to watch this and not feel the same.