Described by some critics as "the best show nobody's watching", The Leftovers is rewriting the rules of television drama.

Launched in 2014, the series introduced a big supernatural mystery that it has refused to completely explain: the inexplicable disappearance of 2 per cent of the world's population, which leaves police chief Kevin Garvey (Justin Theroux) and his family reeling.

Many people think it is the Biblical "rapture", whereby believers are spirited to heaven, but no one really knows. And whereas that sort of ambiguity has landed comparable dramas such as Lost (2004 to 2010) in hot water with fans, The Leftovers has instead leaned into this, morphing into an existential rumination on faith and loss that has attracted a cult following.

As the three-year-old show tees up its third and final season on HBO, which premieres on Monday, creators Damon Lindelof and Tom Perrotta tell The Straits Times they have never been worried about losing viewers with their unorthodox approach.

At a press day in Los Angeles, Perrotta, the executive producer and screenwriter on whose novel the first season was based, says: "One of the interesting things about The Leftovers is it doesn't fit any natural model of genre - it's hard to say what it is.

"It fits broadly into the apocalypse genre, but most apocalypse stories are about survival rather than about philosophical realignments," says the 55-year-old, whose 2011 book of the same name was inspired by the terrorist attacks on Sept 11, 2001, and the 2008 financial crisis.

There is no point trying to solve The Leftovers' core mystery or guess at the writers' next move, adds the scribe, who says his goal was to write about an apocalypse "where everything looked the same, the infrastructure was still there, but all the damage is inside".

"Because we've said we're comfortable not explaining, we're also saying to you that this is not a show about finding the answer or outwitting the writers," says Perrotta, whose previous novels were turned into the Oscar-nominated films Election (1998) and Little Children (2004).

Lindelof - one of the most influential writers in Hollywood and a keen observer of trends in television - has experience with tricky shows such as these: He co-created Lost, a complex story about plane-crash survivors on a mysterious island that vexed fans when it did not tie up all the loose ends.

Asked if audiences are becoming more accepting of series that do not give them all the answers, he smiles. "Well, we hope so."

It helps that "indie TV shows" such as The Leftovers are now more economically viable because of the sheer volume of original programming dubbed "peak TV", as well as the smaller viewing numbers required to sustain pay-cable and streaming-service titles.

"As a result of peak TV, you can create niche shows that don't necessarily have to have massive audiences, but they have passionate audiences and can co-exist alongside juggernauts such as Game Of Thrones or The Walking Dead," he says, referring to the hit fantasy and zombie series.

"And there are these little kinds of 'discovery' shows that don't have many episodes and certainly not as many viewers, but there's a wide range of tastes, so just like indie movies and big superhero action films, they can co-exist.

"We've always been pretty aware that we were making an indie television show," says the 43-year-old, who has also written big-budget films such as World War Z (2013), Star Trek Into Darkness (2013) and Tomorrowland (2015).

With The Leftovers, the idea was never to make a massive commercial hit. "It would've been great if a lot of people were in the audience, but we were never going for the Lost or Game Of Thrones-sized audience and that allows you to be more risky in your storytelling and you can write 'up' to the audience because you're not afraid."

Television viewers today are primed to understand different modes of storytelling, he believes.

"The audience's level of sophistication is the same as it's always been, but the more TV they consume, the more their brain is taught to perceive stories in different ways, so you can skip over the way you do the run-up to something - because, you know, Mr Robot has already done this, you don't have to walk me through it anymore," he says, speaking of the acclaimed hacker series (2015 to present), which also flirts with existential themes.

Lindelof bucked convention again when he announced, after a critically acclaimed second season, that this would be the final season of The Leftovers, with no spin-off possibilities.

Also going against the grain of a lot of must-see TV, he wrote a letter to television reviewers, urging them not to binge-watch Season 3, because "bingeing is bad".

But he promises fans "as satisfying an ending as we can give them" and urges them to trust the writers rather than worry about whether everything will be tied up neatly.

He feels the same way about his own favourite series, including the spy thriller, The Americans (2013 to present).

"If you say to me, "What do you want the final season of The Americans to be?', I'd be, like, 'Just surprise me.'"

•The Leftovers Season 3 debuts on HBO (StarHub TV Channel 601) on Monday at 9am, with a same-day encore telecast at 9pm. Also available on HBO On Demand (StarHub TV Channel 602) and HBO on StarHub Go.