Dublin Murders doesn't give us all the answers – and that's a good thing

• Meet the cast of BBC drama Dublin Murders
• What is the story behind Sarah Phelps' twisty BBC detective drama Dublin Murders?

Screenwriter Sarah Phelps has wrestled Tana French's first two novels, In the Woods and The Likeness, into one drama which follows Irish Garda detectives Rob Reilly (Killian Scott) and Cassie Maddox (Sarah Greene). These emotionally-damaged cops are on a quest to solve two nasty murder cases; by the end of the eighth and final episode, both are solved.

But while we're given motives and confessions and full run-downs of how the killings were carried out, some of the larger mysteries are left unexplained. And that's absolutely OK.

To recap: the finding of Katy's body in the woods of Knocknaree had secret significance for Rob (aka Adam), because that's where his childhood friends went missing in 1985; he was the only one to emerge alive (screaming, amnesiac, with someone else's blood in his shoes) and had to move away and change his identity. Meanwhile, Cassie Maddox was shaken when a woman looking exactly like her was found stabbed to death, and even more unnerved when the victim was identified as "Lexie Mangan" – the name she once used when undercover, which was also the name of her naughty imaginary twin as a child.

After Rob had spent eight episodes chasing ghosts and digging up the past and staring down imaginary wolves, he realised that Katy's murderer was closer to home. The girl's psychopathic older sister Rosalind Devlin had manipulated her boyfriend Damien Donnelly into killing Katy, lying to Damien that their dad Jonathan sexually molested her while cruel Katy laughed at Rosalind's suffering. Damien needed to be her murderous knight in shining armour, she said.

In a way, Rob's instincts were correct – because this horrific crime had roots stretching back to the events of 1985. Jonathan Devlin (then a teenager) had helped his friend Cathal to rape his girlfriend Sandra Sculley in the woods, secretly watched from the bushes by naive kids Adam, Jamie and Peter. When the teens realised they'd been witnessed, they gave chase, and that was the night Peter and Jamie vanished into thin air and were never seen again. Their ugly crime against Sandra poisoned the rest of everyone's lives; Jonathan entered an unhappy marriage with his alibi (Margaret) and had an unwanted child (Rosalind) who grew up twisted and resentful. It all ties together. The past is never really past.

But on that night in the woods, the teens did see the third child, Adam – screaming his lungs out and desperately gripping the trunk of a tree, with unexplained slashes in his t-shirt, blood in his shoes, and unbroken skin. So what strange thing happened to the children in the woods? We're no closer to an explanation. Was it a supernatural event? Was the king of the forest responding to the rape? Do Shaun's black-painted daubings of "HE RISES" actually tap into the truth?

Leaving that mystery open also reflects Tana French's original novel, In the Woods, which ends in the same place as the TV drama: Adam, watching the motorway construction begin at Knocknaree, realising that he may never regain his lost memories of that night. He may never know why he was the boy who returned while his friends disappeared forever. It is a reality he will have to live with, somehow.

As for the Cassie/Lexie storyline, in the end the explanation is somewhat prosaic. A bus passenger who happened to look exactly like Cassie was (understandably) mistaken by an old "classmate" for university dropout Lexie Mangan, the name Cassie had assumed when working undercover a couple of years ago. For whatever reason, this woman saw an opportunity, decided to roll with it, and slipped into the fake identity Cassie had discarded; "Lexie" was then integrated into a seriously toxic group of friends and gifted a portion of a house, but when she tried to leave them behind and sell her share of the property, they (or specifically Justin) killed her.

So who was this woman, before she was Lexie Mangan? We'll never know, because the Dublin Murder Squad draws a blank. But perhaps we also don't need to know; not all mysteries can be untangled. (That said, I'm still not 100 per cent convinced you can have a doppelgänger so similar-looking no one can tell you apart, unless you actually are long-lost identical twins separated at birth. But then again – dramatic licence.)

Dublin Murders leaves an uneasy feeling behind. It's meant to. Sometimes you just don't find out what happened to a missing child or missing person, and it haunts the survivors for the rest of their lives; sometimes you have to accept the seemingly-impossible, in the absence of an explanation.

And sometimes, in a deep dark forest, the mythical becomes believable. As Sarah Phelps put it ahead of the series launch, Dublin Murders travels "into the darkness of how our imaginations are formed, and how we tell each other stories and why we tell each other stories. What is the story going to do? It's going to keep back the beast. We're going to sit round the fire and we're going to tell a story and the beast is going to patrol just at the very edges of the light, and just scratch at the backs of our imaginations."

Consider my imagination thoroughly scratched.

Dublin Murders is available on BBC iPlayer now