In the early Nineties, Her Majesty’s Customs and Excise was losing its battle with the country’s drug gangs. Heroin was flooding in from northern Pakistan, through Turkish and north London crime networks, and onto the streets. Overwhelmed and seemingly out of options, the Investigation Division did something nobody saw coming. At the behest of Margaret Thatcher, they recruited a small team of their own officers, gave them false identities – “legends”, in the trade – and sent them deep into the criminal underworld.

These were not trained intelligence agents. Not police detectives. They were customs officials, used to checking suitcases and occasionally confiscating Scotch. Barely overseen and with limited funding, these undercover narcs ended up seizing more than 12 tonnes of heroin, with a street value of over £1bn.

If this sounds like the plot of a thriller, that’s because it is. Legends, which arrives on Netflix on Thursday, is Neil Forsyth’s gripping six-part dramatisation of that top-secret operation, following those agents as they meticulously build their case by observing – and often infiltrating – gangs in Liverpool and London. Steve Coogan plays Don, a jaded senior officer who recruits a ragtag band of customs employees willing to risk everything. But it’s an imposingly taciturn, denim-clad Tom Burke who anchors the series as Guy Stanton, the undercover officer who finagles his way into a terrifying Turkish drug-smuggling syndicate. Stanton still uses a pseudonym to this day, having betrayed some of the world’s most dangerous criminals. It’s his memoir, The Betrayer, on which the series is based.

“The thing that will really get people is that they didn’t know this existed,” he says. “People are going to look at this and think: ‘My God.’”

The unit, known as Beta Projects, was a small, hand-picked cadre within the service, its members working deep undercover for months, sometimes years. Stanton, the son of a car mechanic and a cleaner, built a cover identity from the ground up: operating out of a council flat in east London, he wore a long ponytail, expensive suits, and a confiscated diamond-encrusted Rolex on his wrist. The character he assumed was that of a fixer – a man who could source anything – and a network of criminal contacts, assembled over years, took him from the grimmest parts of the capital to the poppy fields of northern Pakistan. He did this for more than 11 years, all the while maintaining his life as a husband and father, his family believing he had a quiet office job. “It was a hell of a world to stay in,” the 68-year-old tells me, “keeping that fine line and balance between doing your job and being at home.”

It’s no surprise the series is getting the Netflix treatment. Forsyth, the writer behind The Gold – the BBC’s reconstruction of the 1983 Brinks-Mat robbery and its chaotic aftermath – describes it as “a rich, nuanced true story that has genuine depth and feels unique. I couldn’t quite get my head around it – because for me, like a lot of people, customs officers were men and women who check your suitcase at the airport.” He met Stanton three years ago over lunch, sat with him on half a dozen occasions, and found himself drawn further in each time. “For me, the big thing was that they were essentially glorified amateurs – very smart people leading them, but they hadn’t gone through years of training and selection,” he says. “They learnt in the streets, on their wits. It’s not a polished spy show: it’s something much more interesting.”

Not that the world of covert operations was a safe one. Far from it. The danger was constant, and there were moments, says Stanton, when he thought: this is it. “But it comes with the turf. You should be able to talk your way out of it, or de-escalate.” Guns were everywhere – “They were like an accessory,” he says – and once, during a cannabis haul, four of his colleagues nearly lost their lives when their vessel, battling heavy seas far out in the Bay of Biscay, began taking on water. They were adrift for hours before being found by a Norwegian ship.

Forsyth was fascinated by the minutiae. “Guy told me this story about being on the Tube one day with his family,” he says, “and I just couldn’t believe it.” While travelling with them on the underground, Stanton spotted a Turkish criminal he’d met undercover. “It went into the show very cleanly.” Stanton’s wife and daughter came and watched the filming that day. “It was quite odd for Guy,” says Forsyth. What the show captures above all, according to Stanton, are “the stresses and the on-edge type of thing – always looking over your shoulder.”

Not everything was so simple to incorporate. People talk about creative licence pejoratively, explains Forsyth – “which is kind of illogical to me, because otherwise you should just make a documentary”. In Legends, the plot technically comprises two or three real operations stitched together. Bar Stanton, the characters are composites, most of them inspired by more than one person; some aspects have been changed for legal reasons, others because the people involved don’t want to be found.

Burke, who spent an afternoon with Stanton over a cup of tea, nails the cockney vernacular to an uncanny degree; the ponytail, though, he left behind. “There’s no way I could have told Tom, ‘You’ve got to do this, you’ve got to do that,’” says Stanton. “I think he already had it in his head.” When Stanton and his wife watched the finished series, they were suitably impressed. There’s one piece that he says in there – my wife and daughter just fell about. They said, ‘That’s you to a T. An absolute T.’”

From Stanton’s account, it’s evident that the moments of darkness were balanced by snatches of levity. Back in London, the tracksuit boys – the low-level gang members who did the dirty work – weren’t exactly a model of sophistication, spray-painting their guns to match their outfits. Purple, mostly. “Not really thinking that when you’ve spray-painted a gun, it doesn’t work any more,” notes Stanton, laughing. “A purple shooter might look good. That’s all it does.”

Burke handles the tonal shifts with complete assurance. So, too, does Coogan, whose droll authority gives the show much of its moral ballast, while Hayley Squires is by turns drily funny and stoically determined as Kate, one of Stanton’s fellow operatives.

Stanton wrote the book during Covid. “I never imagined anything like what’s happening at the moment,” he says of the Netflix adaptation. “I wrote it so that my daughter, and now my granddaughter, could look back later and think, ‘Oh, that’s what Dad did.’” Published in 2022, it drew fury from some of his former colleagues. “They said, ‘You’ve told everybody our secrets.’” Stanton laughs. “One person pointed out: ‘What secrets?’ They couldn’t come back with an answer.” He pauses. “I’m waiting for the avalanche [of complaints] to come with the series.”

Does Stanton still feel the effects of the job now? “It affected my health in some ways,” he says mysteriously. “It’s water under the bridge. But there are still moments – silly little things, really. My family always laugh. We go into a restaurant and I’ll sit with my back to the wall in a corner, looking at the door, making sure I’ve got the door in sight. It’s just little things like that.”

He has a WhatsApp group with some of the old legends. Every now and then, someone posts about the biggest drug haul in the world. “And you just think,” he says, “well. It’s not stopping.”

‘Legends’ is on Netflix from today