With The Mole Agent, Maite Alberdi set out to make a film noir documentary about a spy in a nursing home. She did not expect it to transform into an aching meditation on isolation and loneliness.

The Chilean film-maker told the Guardian she was initially toying with genre and form. In early scenes, she makes you question whether you’re even watching a documentary because of the heightened noir aesthetic – venetian blinds and high contrast lighting.

The film, which premiered at the Sundance Film Festival in January, follows the octogenarian Sergio Chamy who is hired by a private eye, Rómulo Aitken, to snoop around a nursing home. Gradually, life chips away at the Pink Panther playfulness of the premise, and the film settles into a sombre look at ageing and desperately hanging on to human connection.

That journey, which for Alberdi is the accumulation of 300 hours of footage, flies by breathlessly in a 90-minute film. It began with a search for her lead. She landed at Detective Aitken’s office. He’s a former federal police investigator turned middle-aged private eye who had already handled four cases in nursing homes.

As a documentary film-maker, Alberdi felt a connection to Aitken’s trade. He waits until he can find proof, just as she waits until the right scene appears before her camera. “I always say my budgets are to pay the crew to wait until the things that we are waiting for happen,” Alberdi said.

They waited at Aitken’s office until the right case came along: a client wanted her mother watched to make sure she was being treated well inside a long-term care facility. These cases, Alberdi explained, are common, driven by the guilt of offspring who leave their parents in such institutions. The nursing home private eye, Alberdi said, is akin to the nanny cam.

Aitken’s regular mole suffered a hip injury. He had to hire a new one. That led to a hilarious casting … erm … hiring process. We see various seniors in the interview chair responding to the help wanted ad. They’re baffled anyone would want to hire someone over 80. But they jump at the opportunity to break their moldy routines and be mentally stimulated with a new gig. The irony that entering a nursing home would be stimulating was not lost on Alberdi.

When it came to that casting process, the documentary director did not take an impartial stance. “I fell in love with Sergio,” she said, explaining that she convinced Aitken to hire the soft-spoken and gentle 83-year-old widower. Chamy couldn’t navigate the camera on an iPhone, which would be deemed an essential skill in nursing home spycraft. The way he fumbles with tech in not so subtle ways lends the film much of its humour. But Alberdi felt he had the right disposition to be the film’s anchor, while embedded for months as the new resident in the senior home.

She already had a crew on scene before Chamy’s arrival. The staff were told that her crew was making a documentary about daily life in a nursing home. That turns out to be closer to what the film became, since Chamy soon grew tired of spying. Despite all the deception, the film ended up being far more honest than intended.

“Our heart was not in the case,” Alberdi said, explaining how her crew followed Chamy’s lead. Though he found his “target”, made contact and regularly checked in on her wellbeing, his undercover dispatches to Aitken grew less frequent. The gentle spy grew resentful of his boss and the covert job. Instead, Chamy befriended other residents and turned his attention to them. And the documentary became about the long-term care home’s social dynamic.

Early scenes show the predominantly female residents watching the cameras suspiciously, reminding each other that those boom mics are listening in on their conversations, as if they too had secret agendas. But eventually they ignore the crew’s presence and carry on, often fawning over Chamy, who was one of only a handful of men in that home.

Alberdi explained that the gender breakdown in this particular nursing home is not common in Chile. Men are usually dumped in homes, while women are kept with the children or grandchildren they raised. However, this particular home goes back decades, with a clientele largely made up of women who never married, had children or found their independence with careers.

Alberdi pegged the home as a portrait of a patriarchal generation. “Your life makes sense if you get married,” Alberdi said, describing the sentiment that led to many of these women arriving at the home in their 50s, deposited there as though they did not belong in society.

In that way, The Mole Agent feels disquietingly similar to Alberdi’s 2016 documentary, The Grown-Ups. That film is a compassionate portrait of young adults with Down’s syndrome who desperately want independence and resent being infantilized. Both films, for Alberdi, are about people that society refuses to integrate. They’re kept isolated because they are deemed too dependent – or rather, too much work.

In The Mole Agent, Alberdi homes in on the micro-society the women and the nurses build for themselves, and how Chamy navigates that. In scenes that alternate between humour and heartbreak, he brings them conversation and comfort. He helps them cope, and when needed, stand on their own two feet. He keeps the home’s resident kleptomaniac in check, returning stolen necklaces to their owners with a gentle nod of disapproval towards the amusing culprit. And, if only for a short while, he entertains the amorous eyes of women like Berta.

“I would consider giving God my virginity through my future husband,” says the simultaneously pious and randy Berta, who’s been in the home for 25 years. Alberdi’s film is particularly attentive to how Catholicism, patriarchy and female sexuality can thrive at once in this environment. That one line from Berta nails it all.

The sadness that throbs throughout the film swells near the end, when the women in the home pour their heart out to Chamy about missing their families and being left behind. With that emotional finale, Alberdi expected her film to inspire a conversation in Chile about the loneliness and depression experienced by seniors. But Covid-19 did that instead.

The pandemic forced long-term care facilities across the globe into intensive lockdowns. The ageing population, who are the most vulnerable to the virus, are now no longer able to receive visitors. Their isolation intensified. But the world started paying attention.

“I read the newspaper on Saturday and there were pages speaking of depression in old people,” said Alberdi.

She sees a silver lining in the current situation. She rarely witnessed a visit or a call from family to residents during her months at the nursing home. But now, during the pandemic, Alberdi said she’s noticed that people are regularly checking in on their parents or grandparents in senior homes.

Seniors, who she couldn’t trust to handle iPhone cameras, are now jumping on Zoom calls with their grandchildren.

• The Mole Agent is available digitally in the US now with a UK date to be announced