We were not expecting that shocking twist in Liar episode 4

And by "that" we mean (spoiler alert) the drugging and raping of DI Vanessa Harmon (Shelley Conn) by Andrew, who turns out to be some kind of psychopath with a long track record of carefully-planned crimes and a cruel, vindictive streak. His politeness and charm, his wide-eyed protests that this was all some kind of mistake – all of that was cover for a serial rapist.

As he crept into Vanessa's house to "punish" her for daring to accuse him of his crimes and turn him down for a date... well, we could barely watch.

From the surgical gloves to the careful washing-up of her drugged-up juice bottle and the methodical unbuttoning of her shirt, it was too horrible to keep looking at the screen.

The Williams brothers set up their ITV drama with a simple premise. Audiences were torn between Laura's anguished insistence that she was raped, and Andrew's certainty that he was the real victim of a false accusation.

They gave us reasons to trust both Laura and Andrew, to the point where viewers suspected a third party was involved. But then at the very end of the third episode, Andrew was suddenly revealed as the "liar" who slipped drugs into Laura's red wine so he could rape her.

The decision to reveal Andrew's crime immediately sent the drama off in a completely different direction. And now we conclusively know he is the baddie – well, what a baddie he is.

Freed from the need to keep audiences guessing about Andrew's guilt, the Williams brothers made things truly dark. While Laura searched the streets of Edinburgh for answers about Andrew's late wife and her "suicide", the man who raped her launched a defamation lawsuit against her and began to target Vanessa.

The first inkling that things were about to go wrong for Vanessa came at the end of episode three, when she bravely told Andrew she knew he was a rapist, and refused to go on a date with him.

The second inkling came with her backstory: a wife away serving in the military, a baby on the way, 10 weeks along and very much a wanted pregnancy. She was just about to transfer departments after coming across too many Andrew Earlhams, which is the equivalent of the "cop on his last day before retirement" cliché.

The third inkling came with Andrew's Instagram stalking and his cruel smile. Oh no.

But still – when he broke into her house, made a copy of her key, and planted the date rape drug in her smoothie in the fridge, it was hard to believe what we were about to see. And when we did see those lingering shots of Andrew taking off her glasses and touching her with his gloved hands, it was utterly chilling.

And then to see her blood in the toilet as she clasped a hand to her mouth, afraid she'd miscarried...

Where can Liar go from here? Will it continue to get darker? Will Andrew strike again? With two episodes left, there's a lot more to unfold. It's going to be hard to watch.

Liar airs on Mondays at 9pm on ITV