One day, retired firefighter Rick Stanton got a phone call at his home in Coventry. It was Ron Howard. He had good news. He had cast someone who had been in Lord of the Rings to play Stanton in his new movie, Thirteen Lives. Stanton’s mind raced. Ian McKellen? Andy Serkis? Probably not Cate Blanchett. “When I found out it was Viggo Mortensen, I was very pleased,” says Stanton, eyes glinting. “I’d never really thought who would play me in a film, but he’d be up there.”

Stanton was one of five cavers (“I call myself a caver not a diver, and definitely not a spelunker because that’s American”) who rescued 12 boys of the Wild Boars football team and their coach from the Tham Luang cave in northern Thailand in June 2018. The boys, aged 11 to 16, had gone into the cave after football practice and got trapped by flood water. They survived for 18 days on an elevated rock two-and-a-half miles (4km) from the cave mouth, with hardly any food but sustained by meditation led by the 25-year-old coach, Ekaphol Chantawong, a former monk. And by hopes of rescue.

One Thai Navy Seal diver had already drowned in the flooded caverns while trying to take oxygen canisters through the narrow tunnels to the trapped group when Stanton had a crazy but brilliant idea. He realised that because the boys had no experience of cave diving, they would need to be unconscious as they were carried through miles of narrow, twisting and claustrophobic tunnels filled with water. So he summoned his Australian friend and colleague Richard “Harry” Harris (played in the film by Joel Edgerton), who is not only a cave diver but also an anaesthetist, to join the rescue operation.

Stanton asked Harris to sedate the boys with tranquillisers before they were extracted, one by one, each accompanied by an expert cave diver on their journey of more than five hours through murky waters in constricted tunnels back to safety. Every 30 minutes or so during the risky return journey, each boy was to be dosed with ketamine to keep them sedated.

Why? “If they came to in the middle of the dive, they would have panicked. Imagine if I blindfolded you and took you into subways and you didn’t trust me and then you took off the blindfold and how you would react. Now imagine that under water …”

So how did he come up with the plan? “The monsoons were coming in and the rest of the caves risked being flooded. We needed to act. Harry didn’t think it would work, but we had no plan B. We thought we were going to be bringing out 13 corpses. Nothing like this had ever been attempted before.”

Other options had, in fact, been considered: waiting out the monsoon season, teaching the boys diving skills, even drilling a new entrance. Elon Musk suggested his mini sub could have carried the boys to safety.

That weird wrinkle does not make the final cut in a film that is not what it seems. It’s a thrilling action movie that also manages moments of astonishing contemplation. It’s a mainstream summer release tilting for a best picture Oscar that will do for Amazon what Coda did for Apple and Roma for Netflix. And it’s a white-saviour narrative that undercuts itself at every turn.

Anyone hoping for a story in which heroic Brits and Aussies save the day while the locals cheer from the sidelines is in for a shock. Howard’s film meticulously shows that the rescue effort involved more than 10,000 people, mostly Thai, but with volunteers flying in from around the world to offer their skills.

“We’ve seen this sort of thing happen before, in the tsunami in Japan, when humanity pulls together,” says Thirteen Lives’ co-producer Vorakorn Reutaivanichkul over a video call from Thailand. “Ron’s a very humanistic director so he wanted to show how people from all cultures and walks of life worked together to make the rescue happen. We really didn’t want another white-saviour narrative. Not just because we’ve seen it too many times in films, but because it isn’t what happened. The film is uplifting about what humans do in crisis, which is, I think, what we need in this modern world.”

When Stanton and his fellow caver John Volanthen (played by Colin Farrell) pitched up at Tham Laung cave ready to rescue the boys, there was some understandable resentment from Thai Navy Seals already on site. In the film, the young Seals complain about two ageing white blokes summoned from the other side of the world to deal with a problem they had in hand. Later, the British duo were joined by other cave divers Jason Mallinson (Paul Gleeson), Chris Jewell (Tom Bateman), Richard Harris and Craig Challen. In the film at least, the Navy Seals and the international cave divers ultimately work together to get the boys out.

William Nicholson’s screenplay illuminated Howard on another, previously untold, aspect of the rescue. While the divers threaded themselves through watery tunnels below ground, above ground another drama was playing out. A water engineer called Thanet Natisri was galvanising local people to help him fill in sinkholes that are flooding the cave network in order to give the rescue mission more time. Howard shows the volunteers who fill in these holes and line the hillside with pipes, some made from bamboo, to divert the water. These scenes echo the Werner Herzog epic Fitzcarraldo, with its human battle against torrential elements, but the twist is that this battle is being fought, not by a vainglorious individual but by a selfless band of locals.

A key moment in the film comes when Natisri tells farmers that, if they are to slow down the water flow that risks drowning the boys, their fields must be flooded with diverted rainwater that will destroy their crops. A spokeswoman agrees and the fields are promptly flooded for the greater good.

One of the most appealing things about Howard’s representation of the events is that it subverts the traditional Hollywood tropes where humans are saved by some gnarly individual with the right stuff, often American and stripped to a stained singlet when he kisses the girl in the end credits. “I hope my film isn’t like that,” says Howard. “I’ve got two models for how people work. One is the family and the other is working on sets of TV and films. Both involve collaboration and problem solving. That’s all I know in life!”

Howard also foregrounds the fact that some of the boys were stateless, belonging to the Shan ethnic minority that lives on the borders of Thailand and Myanmar. “They often don’t have basic rights,” explains Reutaivanichkul, “and it’s important we address that in the film because some of the boys and the coach didn’t have Thai citizenship at the time. Although they do now.”

The film also finds space for the spirituality of locals. There’s a scene showing the devotion of some to a Buddhist priest who visits during the rescue. And another in which local people pay homage to an animistic shrine at the cave’s mouth. That shrine is devoted to the so-called Reclining Princess who animates the mountain landscape. Legend has it that a beautiful princess took her own life after her stable-boy lover was killed by her father’s men. Her body then became part of the landscape that you can see today – the silhouette of the mountain range looks just like a reclining princess. In the film, Howard depicts that silhouette behind the rescuers as they set off into the tunnels.

“These spiritual matters are very live for a lot of people in the area,” he says. “There’s not only karma but for some of the parents what happened to their children was seen as a kind of test of character. One that they passed, incidentally. I went to long lengths to show this without, I hope, being voyeuristic.”

Thirteen Lives has not been screened in Thailand, nor have the real-life boys and their families seen it yet. “We have a lot of obligations that flow out like the water in the caves. We have to honour them,” says Reutaivanichkul. “We have to make a film that shows Thailand beyond the tourist cliches. It’s not just pad thai or muay thai. This is a very specific region in Thailand with a community that we need to show as it is.”

In his efforts to stay faithful to the environment and respectful to its people, Howard deserves substantial kudos. It’s clear why he was drawn to the story, though: before he made A Beautiful Mind and Frost/Nixon, there was Backdraft and Apollo 13 – classics in the disaster movie canon.

People battling seemingly insuperable odds over which they ultimately triumph are his bread and butter “Of course! That makes for better drama.” Would he have made this movie if the rescue had failed? “I don’t know. I don’t think I’m naturally drawn to tragedy. I’m drawn towards the validation of optimism.”

Thirteen Lives bears closest comparison to Apollo 13 (1995), which also depicted a real-life disaster. Astronauts hoping to become the third Nasa crew to land on the moon are confounded by an on-board explosion that depletes the spacecraft’s oxygen supply and power. The mission becomes about saving the lives of the astronauts with a plan hatched by Nasa staff in Houston and the crew. “I was drawn to the cave rescue in part because it incarnated something that impressed me in the early days of Nasa – that the best idea wins. The same happened during this rescue.” Similar, too, is the trickiness of the shoot: just as Howard couldn’t film Apollo 13 in space, he couldn’t make Thirteen Lives in the caves where the boys were trapped.

Howard has never, in fact, set foot in Thailand. “Because of Covid we couldn’t film in Thailand, but I’m sure we couldn’t have shot in those caves anyway.” Set designers built five mock-ups of the caves in Queensland, Australia. Howard tells me little holes were drilled into the model cave walls so that cameras could film underwater sequences. These were digitally filled in during post-production.

More authenticity came from the decision not to have the Thai locals talk together, inexplicably, in English. “The great thing is that now we don’t have to think that way. Because of streaming, subtitles are suddenly OK. We shot the boys and parents speaking not just in Thai but in a dialect that is spoken in northern Thailand. For someone from Bangkok it’s like what a Scottish brogue sounds like to a Cockney. When we tested the movie in very middle-of-the road places, the audiences not only weren’t baffled by subtitles, they kind of expected them.”

As for Stanton, what did he think of how Mortensen played him? “I think he nailed my accent,” he grins. Yet Mortensen hardly speaks a Coventry accent. “But I’m originally from Essex,” says Stanton, “so that’s in there as well.”

What’s yet more impressive is how Mortensen plays Stanton as undemonstrative, focused on the job, selfless, never grandstanding – as if channelling the antithesis of Boris Johnson.

I ask how Stanton has adjusted to life as a global hero. He chuckles in a way that Mortensen would not in the film, but then adds phlegmatically. “I don’t think of myself as a hero. I still live alone in Coventry.”

• Thirteen Lives will be in cinemas on 29 July and Prime Video on 5 August.