NEW YORK (NYTIMES) - Documentary film-maker Elizabeth Chai Vasarhelyi lives in fear of not telling a complete story. What if there is another angle to explore? More footage to uncover? Is her exploration of a topic ever really complete? Those feelings were occupying large swaths of her brain back in May when she was finally able to travel to Thailand.

Ms Vasarhelyi, 42, and her husband, Mr Jimmy Chin, 47, are best known for their Oscar-winning, death-defying climbing documentary, Free Solo.

The duo had already spent three years painstakingly turning over every piece of video available to them for their new film: The Rescue, which opens on Friday (Oct 8) in theatres.

The film tracks the 2018 global effort to retrieve 12 young soccer players and their coach trapped in the flooded Tham Luang cave in Chiang Rai province, Thailand.

The film-makers had scoured international news feeds and local Thai footage, often piecing together scenes from a slew of disparate sources. What they could not find, Ms Vasarhelyi and Mr Chin as well as the British divers who led the rescue operation re-created in a tank in Pinewood Studios in Britain.

They had essentially completed their movie. It was moving and harrowing, yet it still nagged at Ms Vasarhelyi. It was missing the scope of the operation and some smaller, more intimate moments that underscored the gravity of the situation.

But those moments were in the hands of the Thai Navy Seals, and after two years of negotiations, no amount of effort on Ms Vasarhelyi's part had convinced the military to share the footage with her.

Until May. When Ms Vasarhelyi, fully vaccinated and willing to endure a two-week quarantine in Thailand, made the trek to Phuket to meet with Rear-Admiral Arpakorn Youkongkaew, a Royal Thai Navy Seal commander, and his wife Sasivimon Youkongkaew, a former television journalist who had the instinct to give the Seals cameras at the beginning of what would become an 18-day rescue operation.

"We spent three years with this story - I'd be writhing on the floor if it popped up" after the film was finished, Ms Vasarhelyi said, referring to any missing scene. "It's like the code of non-fiction: If it's out there, we have to try everything to get it."

This time, after a long meeting when Ms Vasarhelyi again conveyed her intention to include all sides of the story, they finally agreed. She returned to the United States with the promise of a treasure trove of footage and the assistance of Mrs Youkongkaew, who flew to New York with the 87 hours of footage in her backpack and the patience to sift through it.

"It's like a dream come true for a nonfiction filmmaker. It was also a nightmare," Ms Vasarhelyi said about the arrival of all that footage after their film was supposedly finished.

The result of that extra effort is a visceral, heart-thumping cinematic experience, as edge-of-your-seat as American rock climber Alex Honnold's journey in Free Solo, even though the fate of the soccer team had been well-documented.

Fifteen minutes of footage from the Seals (and the Thai army) is now in the movie, providing the film with an extra layer of scope. Thanks to the rescue team cameras, viewers will see the first time divers Rick Stanton and John Volanthan emerged from the cave having found the boys as well as shots of hundreds of people lifting stretchers containing the children out of the water.

"That stuff finally gave you a scale," said Ms Vasarhelyi, who admitted not understanding why so many people were required for the rescue until she saw the footage and did her own cave walk on her trip to Thailand.

The Rescue made its world premiere at the Telluride Film Festival in early September. Three weeks later, when Ms Vasarhelyi and Mr Chin sat down for an interview, the movie had changed again - an additional minute had been added to highlight other crucial rescue tactics.

"The process of this has been so intense," Mr Chin said. "We do want to represent what was really important, and we've been digging at this thing for three years trying to make it right."

Complicating Ms Vasarhelyi and Mr Chin's efforts was a complex and convoluted grab for the life rights of the people involved in the rescue. The film-makers were initially attached to direct for Universal, which planned a dramatised version based on the soccer players' stories. But rights to those stories disappeared after the Thai government got involved. Netflix then scooped them up and is currently shooting its own miniseries in Thailand.

For The Rescue, National Geographic, which financed the film, had the rights to the British divers, a ragtag group of mostly middle-aged men who happen to be the best amateur cave divers in the world. While the rescue effort was a global one, without the divers the boys probably would not have survived.

Ms Vasarhelyi and Mr Chin did not have the boys' rights, so she was not permitted to interview them for the film. She did get to meet them when she visited Thailand.

"It wasn't on camera," she said. "I just wanted to hear…and understand," she said.

Ms Vasarhelyi shared meals with some of them and learnt more about their 18 days underground.

She was taken by their role-playing exercises in which one child would pretend to be the parent so the others could re-create the families they were missing. The kids also asked Ms Vasarhelyi to show them the footage she had of them being sedated by Dr Richard Harris, an Australian anaesthetist and cave diver who made the crucial - and controversial - decision to inject them with a mixture of Xanax, Ketamine and Atropine so they could be transported one mile (1.6km) underwater for about 2.5 hours without panicking.

"It was just surreal," Ms Vasarhelyi said. "Of course, they wondered what it all looked like. Of course, they wanted to know what happened when they were under. I'm happy that we were able to share that with them."

The divers were also drawn to Ms Vasarhelyi and Mr Chin's dedication to accuracy. Producer P. J. van Sandwijk, who secured the rights to the divers' lives in two separate deals - one for the documentary, another for an upcoming feature directed by Ron Howard - said the men were initially "apprehensive to do anything".

He added: "They very much came back from Thailand with a mindset of 'this was a global rescue; there were thousands of people on the ground'. They didn't want this to become just about those guys."

So when Ms Vasarhelyi and Mr Chin asked the divers to join them at Pinewood Studios to re-enact the underwater scenes, the men took it as a sign of the film-makers' dedication.

"What we wanted to do all along when we started the documentary was to sort of demonstrate what we actually did and what we went through when we were rescuing the boys," said one of the rescue divers, Mr Stanton, 60, a retired British firefighter.