HERE’S the film pitch. There’s a commercial diver at the bottom of the North Sea on a routine maintenance dive. He’s relatively inexperienced. He doesn’t know it, but a computer error means his support vessel has started to drift off station pulling the diving bell with it. That’s just the start of his problems. The line that attaches him to his diving bell suddenly becomes snagged on an underwater structure.

Snagged is bad. Severed is worse. When that happens, the diver is suddenly stranded on the seabed without light or heat or much air. In fact, his emergency air tank contains five minutes of oxygen. And as for help? Well, help is now 30 minutes away.

Did we mention it’s a true story? A true story and now a film too.

Last Breath, a film by Alex Parkinson and Richard da Costa, is premiering at the Glasgow Film Festival next month. It’s a film that tells the real-life story of saturation diver Chris Lemons who in September 2012 was stranded on the seabed some 90 metres beneath the surface. The film explains what happened next (no spoilers) through a compelling mixture of archive footage reconstruction and interviews with the diver’s colleagues.

It’s a story that co-director Da Costa first heard in a pub near Aberdeen. When he heard it, he says, his jaw was on the floor. “I told Alex the story and it had the same impact on him. We thought if it’s having this effect on us it’s got to be a story really worth telling.”

“It’s such an unknown world,” adds Parkinson. “I knew nothing about saturation diving at all before. Everyone could identify with the horror of being in that situation. It’s such a primeval fear; being trapped and on your own. It’s practically a horror film in a sense.”

Parkinson has a background in making doctumentaries for the BBC, National Geographic and Animal Planet. Da Costa was originally an actor.

“Richard and I first met while I was making a programme for BBC Three about people living as animals to get into animal behaviour,” Parkinson explains.. One of the programmes was about pigs and one of the people living as a pig was Richard. I was filming Richard snuffling around a pig sty. You got on very well as a pig,” he says to his film-making partner. “I absolutely loved it. Again, it’s an extreme environment.”

Extremity, then, might be a defining motif for the duo. Certainly Last Breath is very interested in the subject. The film is full of suspense and exposed emotions (helped by an impressive score from Paul Leonard Morgan), but more than that it gives a glimpse of the realities of what is an extreme job. Here is a vision of men who go up to 150 metres underwater to walk on the seabed in pitch black conditions for eight hours at a time. “What astonished me was how this extreme environment becomes completely normal to these people,” Preston suggests.

“I think it’s typical of divers to be quite extreme personalities,” da Costa adds. You have to be to endure the lives that they lead.”

That comes across in the interviews. Everyone who speaks is open and honest. Some find talking about the situation difficult or emotional. Others are more matter of fact and to the point. That particularly applies to Lemons’s fellow diver David Yuasa who calls himself “The Vulcan.”

“His brutal honesty is just really refreshing actually,” says da Costa. “They’re like a family, the divers. They have to be pretty close to be able to make it work and I think that really comes across.”

Conducting the interviews, the film-makers covered the events of that night in forensic detail. “I think they saw it as a cathartic process,” suggests da Costa. “I think they learned a lot about themselves from being interviewed in such detail.”

Last Breath screens at the CCA on February 28 at 8.30pm and again on March at noon 1. Tickets for all Glasgow Film Festival events go on sale next Monday. Visit glasgowfilm.org/glasgow-film-festival for more information.