The French have done it again, striking cinematic gold with another unlikely film. First it was The Artist, a love story embraced by audiences and showered with Oscars even though it is silent. Now an irreverent film about disability has broken box-office records across the world.

Just as the Paralympics are changing perceptions about disability, cinema is pushing at the same boundaries with Untouchable, an uplifting comedy about the friendship that develops between a wealthy quadriplegic, Philippe, and his carer, Driss, an ex-convict.

Audiences are flocking to see it. It has sold nearly three times as many tickets worldwide as The Artist, though it has not yet opened in several major territories – including the UK, Japan, Australia, Scandinavia and Latin America. Box-office takings are a staggering $366m so far, compared with The Artist's $134m for its entire global run. Made for just $10.5m, Untouchable is topping national charts, including those of Switzerland, Spain, Italy, South Korea and Germany, (where it has become the highest-grossing French film) and figures exceed those for blockbusters such as Harry Potter in some countries.

Based on a true story, the friendship between the two men develops after the millionaire's paragliding accident. The Senegal-born ex-convict becomes his carer despite stealing a Fabergé egg during his job interview, and the pair are drawn together by honesty and humour, fear and friendship. Hollywood producer Harvey Weinstein, who spotted the global potential of The Artist, loved Untouchable so much that he snapped up the rights to an English-language remake, which will star Colin Firth, before it was even finished.

Untouchable is both heartbreaking and, at times, very funny. The two men discover that they both have disadvantages in life – one a physical disability, the other socioeconomic. The film confronts the emotional and physical implications of paralysis, and the way that society makes disabled people invisible or "untouchable".

It has struck a chord with the public perhaps because we live in a time of global crisis, says Eric Toledano, who wrote and directed Untouchable with Olivier Nakache. He says that the film redefines our heroic image, from yesterday's "superhero" to "two simple people who can accept the fragility of their lives". He adds: "We just tried to make love with this story ... the humour saves these two people. It's a kind of British humour. The British can joke about everything, even misery."

The filmmakers struggled to interest investors; there was nervousness about disability. One financier even asked whether the character could "walk a little bit". However, the French companies Gaumont, Canal+ and TF1 saw the potential.

Inspiration came to Toledano and Nakache after they happened to watch a documentary about Philippe Pozzo di Borgo, who was left a quadriplegic after a paragliding accident, and his Algeria-born carer Abdel Sellou. The directors and actors visited Di Borgo, who insisted that they made a comedy and went on to act as the film's adviser.

Portraying a man paralysed from the neck down was a particular challenge, says François Cluzet, one of France's most respected actors. "I'm an actor who isn't fond of dialogue and who loves to act silently. That means I usually need my body to express things … Driss becomes my body in a way."

Toledano adds: "All the acting is in the eyes. It's very hard."

As soon as he read the script, Omar Sy, a rising French actor who won a Cesar for this film, responded to it. His own story is a rags-to-riches fairytale. He says: "I think it's a very human story, and it's funny and touching … Philippe chooses Driss as his caretaker partly because he doesn't see him as disabled or pity him. He just sees another person."

Toledano has been overwhelmed by reactions from disabled people, including those associated with The Christopher & Dana Reeve Foundation, dedicated to curing spinal-cord injury and set up in memory of Hollywood actor Christopher Reeve who was left a quadriplegic after a horse-riding accident.

Peter T Wilderotter, its president, says of Untouchable: "Most times movies only display the despair of disability and do so in a way that doesn't portray the day-to-day challenges. This movie tells the story with truth and integrity and humour towards the ignorance that those who live with paralysis face each and every day. It is a true gift. It's as if Chris and Dana collaborated on the film."

Toledano has received more than 3,000 thank-you messages from wheelchair users all over the world. "You will cry if I show you all the messages I've received," he says. "I am so proud."

• Untouchable is released on 21 September.