Unforgotten creator Chris Lang explains THAT series three finale twist – and why it makes so much sense

"Are you admitting that you killed them? Both of them?" asks DCI Cassie Stuart (Nicola Walker).

"I am. And if we can do this properly, if we can do this with respect, if we can avoid turning it all into some sort of sordid circus – I'll tell you where the others are buried too."

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Gasp! Who saw that twist coming? We've all been wondering about one girl's murder, but now suddenly Hayley Reid is part of a much larger and even darker story.

Suddenly the respectable country doctor's disguise is off and he reveals himself as a rapist, a serial killer and a psychopath whose crimes go well beyond one or two victims. Buried around the country are the remains of untold numbers of adolescent girls who he raped and killed over many decades.

"It's surprising territory for Unforgotten, which is why I wanted to do it," Unforgotten's creator and writer Chris Lang tells RadioTimes.com. "The murder in the first series was a sort of twisted crime of passion, you could argue. The second series it was a revenge, clearly, from psychological damage - a revenge murder. And they both have an emotional starting point, they come from a kind of humanity, in a funny kind of way.

"In this, I wanted to write about murder that is about evil, really, because it felt very very different, and it's much harder to understand, which was what those scenes were all about; listening to someone talking extremely dispassionately about the most heinous crimes."

He adds: "I tried very hard to mask it in his character whilst at the same time creating a character who, when the dust settles, you could believe had done that."

After the confession, the man sitting in the chair opposite Cassie and DI Sunny Khan (Sanjeev Bhaskar) is suddenly very different to the one we've got to know over the last five and a half episodes. Gone is the loyal friend, the caring doctor, the loving father – and in his place is psychopath who says he feels no guilt or emotion. His lifelong pretence is over.

So how can this be? How can Tim be a psychopathic serial killer?

"Most of the ones that we know, led actually fairly humdrum fairly ordinary lives and did not stand out as particularly unusual," Lang explains. "Real proper evil rarely comes wearing a cloak and a dagger. It's wearing a cardigan or a fleece."

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Looking back, you can see some of the red flags. Dr Tim Finch was accused (but cleared) of verbally abusing an elderly dementia patient when he thought she was home alone, taking pleasure in inflicting emotional pain. His ex-wife also said he subjected her to a years-long campaign of emotional and physical abuse that left her terrified.

But this is also a man who helped his friend Chris (James Fleet) through his mental health troubles and gave him the financial aid to get back on his feet. He remained close to his childhood friends, he was a good dad to his daughters, and he counselled vulnerable patients through grief and pain. So why nurture those relationships?

"Because they help you fit in," Lang explains. "They mean that you can hide in plain sight, and the research suggested that people like Harold Shipman – he killed upwards of 300 people probably, and yet he was a pillar of the community, because being a pillar of the community enabled him to do everything that he did. And suspicion would never fall on him.

"Similarly with Dennis Rader, the American serial killer. He was a church leader, he was married, he had three children, he helped out in the local community at every opportunity, and by all accounts was an absolutely model citizen. And he was all of those things.

"Not, I suspect, because he gave one single f**k about any of those things, but because they enabled him to do the other thing that was his primary life motivator, which was murder."

As part of his research, Lang became deeply interested in psychopathy and serial killers – reading books and watching footage of interviews and courtroom testimonies.

"They're appalling, clearly, but they're also fascinating because they take us to a place that we find very, very hard to understand," he says.

So why does Tim finally start to come clean? When Cassie says the police have enough evidence to charge him with the murder of two girls, why does he tell them about the rape and the other bodies?

Lang explains: "The game is up at that point. He's done a really good job of batting back all of their enquiries, all of the evidence that they're slowly amassing, but there comes a point. He's a smart guy, he's a very smart guy.

"The car, he knows that probably Sunny Khan is absolutely right about - that he didn't clean out underneath the spare wheel, and they probably will find 18-year-old specks of blood or hairs or carpet fibres or something that will link that car to her, and therefore him.

"He knows that the chances of him having been in that hotel and that the murder victim died 200 yards from it are just so vanishingly small – that he could be innocent, and that the coincidence happened – it's so vanishing, he knows that the game is up."

So the only way for him to regain control of this situation is to tease the police with something they want: information.

As Lang explains: "At that point, he thinks, 'okay, well I've been kind of waiting for this moment all my life.' He thinks, 'I've had a good run.' He'd been probably killing for 25, 30 years, and at that point he thought: 'Okay, I've lost control of my liberty, but I do still have control of the information that I hold.'

"He's going to start spooning it out."

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