Stairs leading to a basement. An empty fairground ride. A fallen soldier. The opening credits of our psychological thriller The Missing on BBC1 — the last episode of which airs on Wednesday — are full of fragmented clues planted between atmospheric shots of trees and heavy skies.

These subliminal moments last less than a second — just long enough for the brain to register, though perhaps not fully digest.

To viewers who have followed the series since it began last month — relating the story of an abducted girl, Alice Webster — most of these images will now make sense: the child twiddling her fingers and the road seen through the smeared window of a camper van.

The final episode of the second series of the psychological thriller The Missing will air on BBC1 on Wednesday

Most of the images have a number of meanings. Could the fallen soldier be Daniel Reed, shot in combat in Iraq fighting for the peshmerga? Or Adam Gettrick, a former soldier, now the military press liaison officer and morally ‘fallen’? Or something else entirely?

There’s also a frame that bears the number ‘3’. Does this stand for the three girls abducted? The three men we saw in this week’s episode, Gettrick, Henry Reed — Daniel’s father — and Adrian Stone? Or, again, is there another meaning yet to reveal itself?

Some viewers might have wondered over the past few weeks whether we really had a plan when we wrote this second series of The Missing or if we were simply making it up as we went along! The credit sequence hopefully shows that we agonised over every detail and argued about each possible piece in the jigsaw.

The brothers behind the hit series have revealed all - from the true crimes that inspired them to their trickiest plot twists

This is not to say we thought of everything. One of the biggest pleasures of seeing our work reach the screen, after two years of writing and filming, is to read the countless fan theories buzzing around the internet. People have been airing ingenious solutions on social media, some so clever we wish we’d thought of them.

British actress Abigail Hardingham stars in the drama

It’s fantastic that people have become so engrossed in the story. It happened during the first series, in 2014, and we feared that, second time round, we wouldn’t be able to recreate that excitement.

To see people getting involved once more has been a joy and a relief. Viewers are confused — but in a good way. A way that makes people think, which these days is no bad thing. This is a show that demands you aren’t half-watching with one eye on the weekly shopping list on your iPad.

It’s a reminder of how everyone used to watch TV — or at least how we did growing up together as brothers.

As we approach the end of this series, we’re reminded of the ending of series one — which did divide viewers. We left James Nesbitt’s character, the grieving father Tony, frantically knocking on doors in Russia, desperate to track down his son.

Our intention was to show this as the final tragedy: he could not cope with his loss and would drive himself to his grave rather than face the truth that his son was dead.

It was a deliberate choice to leave just a sliver of uncertainty. The boy Tony accosts asks him in Russian who he is, but because Tony doesn’t understand, we chose not to subtitle it.

Roger Allam as Adrian Stone and Tcheky Karyo as Julien Baptiste. The writers have taken on board feedback from the first series and said they will be 'no ambiguous ending'

As a result, it’s possible to view the ending in the same way Tony does — he can only live in a world where he believes his son could still be alive, even if all the signs are that it couldn’t be the case.

Though we’re proud of that ending and stand by it, we listened to the audience who invested time in the series. We took note of the feedback and decided it was our job to write a clear-cut ending with no ambiguity for series two.

To BE truly satisfying, it would resolve all the questions, while leaving room for something characterful and credible that said something about the journey we’ve all just undertaken. So if you’re looking for answers, we promise you’ll get them.

That’s not to say the final episode is all about tying up loose ends. Explanations are fine as far as they go, but ultimately this has been a show about characters: dogged French detective Julien Baptiste, the fractured Websters (David Morrissey and Keeley Hawes), brittle Eve Stone and her father, who harbours a terrible secret.

The show has been about the characters: Dogged French detective Julien Baptiste, played by Tcheky Karyo, left, and the fractured Websters, played by David Morrissey and Keeley Hawes

There are surprises, but no big revelation. We already know that Gettrick (Derek Riddell) is a kidnapper and murderer. To the most astute eyes, that didn’t come as a total surprise. He had been a fringe character until then and some had commented online that you don’t hire an actor as good as Riddell to be a glorified extra.

But we hope the way he betrayed himself as a psychopath — killing lovelorn policeman Jorn with a power drill to the brain — was utterly unexpected. We wanted that to be a leap-out-of-your-seat moment.

There will be surprises to come in the final hour. One of the biggest questions really is life or death for the central figure, Detective Baptiste (Tcheky Karyo), who has been suffering hallucinations caused by a brain tumour.

Everyone wants to know if he’ll survive and whether he’ll be back for a third series. We can understand that because we love the character and the actor, too. Karyo is as charismatic and charming in real life as he is on the screen.

Laura Faser plays Eve Stone, a sharp and straight-talking Sergeant in the Military Police

But the story comes first and we would never want to compromise that for the sake of another series. We felt that strongly in writing the second series: we only wanted to do it with a story worth telling.

Several real-life cases served as inspiration and persuaded us that The Missing II was a worthwhile tale.

Perhaps the two biggest stories that influenced us were accounts of the Ariel Castro abductions in Cleveland, Ohio, and the 18-year ordeal of Natascha Kampusch in Austria.

These cases were so bizarre that if we’d made them up, they would have been dismissed as ridiculous.

Castro kept three women locked in his basement, just streets away from their own homes, and invited his friends into the house to play music and jam with him while his prisoners were silent, terrified captives under the floorboards.

The writers were inspired by real-life cases including that of Ariel Castro, left, who abducted three women and kept them locked in his basement in Cleveland, Ohio; and the 18-year ordeal of Natascha Kampusch in Austria, who was kidnapped as a child

Ten-year-old Natascha was snatched on her way to school and kept prisoner in a cellar.

In another case that influenced us, a confidence trickster convinced a family in Texas he was their long-lost son, despite the fact his hair and eyes were a different colour. He even had a French, not an American, accent.

The parents were desperate to believe him and, as we have tried to show, desperation can play bizarre tricks on the mind.

All of this convinced us that writing another series of The Missing would not be a cynical exercise.

The truth about a third series is that while we would never say never, we don’t yet have the story we feel needs to be told. If we decide to do it again, it would only be because the idea could hold its own against the first two series.

But whatever we do, we’ll write it together. Being brothers is an advantage when it comes to writing because we understand each other — there’s a shorthand. It also makes it easier to be blunt about each other’s ideas. Families are allowed to be mean to each other, right?

Brothers Harry and Jack Williams come from a family of writers and storytellers

We grew up in Putney, South-West London, with a writer for a father and a TV producer for a mother. Our older brother, Ned, is a TV director.

In our house, everyone talked about books and stories the whole time. It made the idea of writing feel like something anyone could do —which it is.

Before writing together we played in a mediocre band with many less than mediocre names. We then wrote comedies for years, though ‘comedy’ is a strong word for what we did.

You might suppose we tackled comedy first because it’s easier. It’s not: you have to make the characters and story work in the same way as drama, but then you need to make it funny. And everyone disagrees over what’s funny . . . especially us.

Our shared fascination with the darker side of human psychology made eerie drama more inviting. We want to put viewers into the shoes of people in extraordinary situations, forcing them to ask what they would do, how they would behave.

To intensify the sense of loneliness around the characters, we set both series of The Missing abroad.

The series is set in Germany to 'intensify the sense of loneliness around the characters'

It plunges everything into an alien environment — one child disappears on holiday, another while her father is stationed with the British Army in Germany — so both have been taken away from home even before they are abducted. For the parents, because they’re outsiders, the place starts to feel hostile. They’re stranded, and that can feel suffocating.

We made a deliberate decision, too, that the German military base should feel cold and bleak, while the scenes in the Iraqi war zone were searingly hot. There are soldiers in both places, but they are different versions of hell.

Eight episodes might seem a lot, but the truth is the scripts were even longer than that. A great deal had to be cut: mostly details that helped weave different strands together.

One of the biggest cuts is the story of how Sam and Gemma Webster’s son Matthew (Jake Davies) fell in with the vicious German twins, wannabe rap stars Ulf and Axel.

It all begins with Andreas (Sebastian Urbanski), a young man with Down’s syndrome who works at a butcher’s shop. As he walks home each day, he passes the house where the twins live.

Anastasia Hille plays Celia Baptiste while Tcheky Karyo is one of the lead characters, French detective Julien Baptiste

They taunt and bully him until he blurts out something about the butcher, Kristian Herz, who has been arrested on suspicion of abducting Alice. Ulf and Axel break into the shop, wearing animal masks, and beat the butcher’s wife, Nadia, into a coma.

At the hospital, Matthew sees Andreas and asks why he’s taking flowers to Nadia. Andreas tells him that she’s his employer and he feels guilty, as if it’s his fault the twins attacked her.

Instead of going to the police, Matthew covers up for the youths and so begins their toxic friendship.

Sometimes people feel these kind of narrative strands are red herrings designed to throw people off the scent. In truth, everything in the show serves a purpose.

The German military base setting was designed to feel cold and bleak - unlike the searingly hot scenes in the Iraqi war zones from the first series

Not everything is related to the central mystery, of course, but it all helps to sow the seeds for what’s to come.

We wanted to make viewers feel Matthew’s attack on Brigadier Stone in the nursing home was inevitable. The assault isn’t a plot device, it’s the logical conclusion to the choices Matthew has made in his life.

Other explanations can be squeezed into the story. In the first episode, Brigadier Stone (Roger Allam) made a throwaway remark to his heavily pregnant daughter Eve (Laura Fraser), commenting that her sister would never forgive him if he failed to ask how she was.

There are soldiers in both places, but the writers say the two locations 'are different versions of hell'

Why would the sister feel so strongly? The answer was revealed four weeks later, when Eve gave birth as a surrogate mother, handing the baby to her sister Anna.

Some viewers had already guessed this twist, because they caught the earlier hint.

A story like this is not about building to the central conspiracy, but it goes a long way to explaining the character of Eve. And the theme of what a parent will do for a child is central to the show.

The speculation about The Missing has been brilliant to see and part of the joy of the show is to build the many clues. Fleeting remarks that pay off episodes later, words on a gravestone, a photo in Brigadier Stone’s office that would feature heavily . . .

The final episode won't leave viewers on tenterhooks, the writers have promised

But ultimately, the final episode is less about pulling the rug and trying to out-do ourselves with plot twists and reveals.

It was always our hope to write a series that began in an uncompromisingly complex fashion, but as it developed slowly came more and more into focus.

Perhaps the biggest surprise that people will find in episode eight is how this multi-layered puzzle is, in the end, an emotional hour of drama.

In and around the theories and surprises, hopefully people will realise they’ve come to love these characters — and if not love them, at least maybe understand them.

All good things must come to an end, and thankfully — for some of these characters — so must all bad things, too.