Nazo Dharejo had barely mastered the alphabet when her father, Haji Khuda Buksh, first showed her how to load a gun. The kalashnikov would be kept on the wall, hung above the living quarters of the family’s two-storey home, where she grew up with her two sisters, and their older brother Sikander in rural Sindh, Pakistan. They were comfortable, but not extravagantly well off; Khuda Buksh worked as a farmer and had inherited a few dozen acres of land from his own father. His wife, Waderi Jamzadi, raised their children and, once the girls left school, aged seven, taught them what she could at home.

The girls were moulded to be tough and resilient. Their father would dress them in trousers and shirts – “boy’s clothes” – instead of more feminine, traditional shalwar kameez. Nazo, the eldest daughter, was given the male nickname Mukthiar and was the first to be taught how to shoot, when she was 16. Two years later, with her brother murdered and her father in prison, the fierce but waif-like teenager was armed and leading a gunfight against a criminal army of bandits sent to steal her family’s home and land. Now 41, she has been dubbed “the toughest woman in Sindh” by the Pakistani press, and the story of that night has become local legend – one that British Pakistani film-maker Sarmad Masud has beautifully rendered in his debut feature, My Pure Land.

Film still from My Pure Land showing Nazo with the stolen bullets. Photograph: PR

“I originally wanted to make a film in Pakistan about police corruption, like Cop Land,” says Masud, in an East Midlands accent. “Then in 2013, I came across a story of this woman who had defended her home and land from 200 bandits. That’s much better than a Pakistani Cop Land so I contacted [the journalist] who wrote the piece and through that, rang Nazo.”

Their first chat set the tone for their relationship: to the point and with minimal conversational faff. “She asked if there would be song and dance in the film and I said no. She asked if it would be a documentary; I said no. She asked if she would be in the film and I said: ‘No, we’ll get someone to play you.’ We stayed in touch on Skype; quite quickly she signed her life-rights agreement but refused – and still refuses – to take money for it. She just said: ‘Tell my story.’” And so Masud has.

My Pure Land has, in many ways, the elements of a masala western: a long, dramatic standoff, where Nazo, her sister, her mother and brother’s friend, Zulfiqar, are under siege for 24 hours in a dusty fortress-like building, exaggeratedly outnumbered by flinty-eyed, grimy villains sent by her father’s brother. Yet, Masud’s tiny-budget film has stripped the scenes and most importantly the dialogue to a minimal, naturalistic rhythm. Yes, there is lush cinematography – the country’s unique light and landscape is an obvious gift to film-makers – but the atmosphere is deliberately toned down with an ambition to do “Paul Greengrass meets Ken Loach in Pakistan”.

Family disputes over land are many and complex in south Asia; there are an estimated 1m cases pending in Pakistan alone, and deeply savage feuds can sometimes tip into violence when honour is at risk. In Nazo’s case, her family’s honour was the land itself, and protecting it was more important than her life: once it was left to the women of the household to hold tight and keep fighting, they decided surrender wasn’t remotely an option – the only way their bodies would leave the land would be together, dead.

In a romantic, albeit sweetly optimistic, move, Zulfiqar asked Nazo that night if she would marry him – if they ever made it out. She said yes under three conditions: that she would carry on working, she wouldn’t live with her in-laws, and that she would be free to travel where she wanted.

“I wasn’t scared,” says Zulfiqar, over the phone. “At that point, fear runs away. Nazo was the same.” The couple went on to have four children, to whom they have told the story countless times. Their son “has been holding a gun since he was three years old, but the girls aren’t interested”. And Nazo, headstrong and independent, has travelled over the years – going on religious pilgrimages to, typically, some of the toughest places in the region: Iraq, Iran and Syria. At this point, the couple have yet to see the film; Masud wants to try and get them over to watch it together on a proper cinema screen. That it even got made and is complete, 16 years after he graduated in television and film design at the Hull School of Art and Design, is still something of a revelation to him. Everything that could have gone wrong with the shoot, he says, did. The majority of funds were raised from family and friends and he had a six-week filming schedule to get it all wrapped up.

“A week before we started filming, we still hadn’t found the actress to play Nazo. My wife [Caroline Bailey, the film’s production designer] was hospitalised with carbon monoxide poisoning, working on the prison set we’d built, the first day of shooting. And after scouting Google Earth and having an absolutely useless fixer, we only managed to find the house we made the whole film in because of my Uncle Riaz. He took us to a property, a few miles outside of Lahore that my grandfather had built himself decades before.”

Masud is animated about the perils of filming in Pakistan, where “anything is possible but everything is impossible”, but he is sentimental about the experience, too. “I want to make more films from there. This is a universal story. It’s not just Pakistani, or a western, or a feminist film.” He tries to explain the emotional pull he feels towards the country. “I’m from Nottingham and I’m proud of that. I moved to London in 2012, but I won’t wear a scarf and I won’t drink coffee. I don’t know where I’m going and I’ve still got a Nokia. But Pakistan ...”

We consider the romanticising that we, as diaspora kids, can be guilty of, but still, Masud says he feels an innate urge to make work that rejects the cliches about Pakistan. “The only real request that Nazo and Zulfiqar made was that I didn’t portray them as terrorists.” He laughs. “But for me, it’s like my dad says, Pakistan to him isn’t just the people and the noise and the language – it’s the soil itself.”

• My Pure Land is released on 15 September