Warning: this article contains spoilers for episodes 1-5 of The Undoing.

After five weeks of dramatic revelations, debates on social media and staring at Nicole Kidman’s hair, the final episode of The Undoing is almost here.

The series, which stars Nicole Kidman and Hugh Grant as Grace and Jonathan Fraser, a privileged couple who find themselves at the centre of a murder investigation, has attracted viewers from all over the world with its mind-boggling plot and mysterious appeal. And after last week’s episode left things on a cliff-hanger, it’s down to the final episode to unveil the truth in this brutal game of whodunnit.

To quickly recap where things left off in episode five: after a number of drama-filled court appearances and Jonathan’s revelation that he had – indirectly – been responsible for the death of his younger sister when he was younger, Grace uncovers the murder weapon used to kill Elena – a pretty nasty-looking sculpting hammer – in her son Henry’s violin case.

Up until that point, the murder weapon used to kill Elena had been missing and the only evidence the police had connecting anyone to the scene was Jonathan’s DNA – so what could this discovery mean for the case as a whole? 

Could Henry be hiding the weapon for someone? Or could he somehow be responsible for the murder?

Basically, episode five raised a whole load of questions we hadn’t even considered a possibility throughout the rest of the series – and it’s prompted fans of the show to share their thoughts ahead of this week’s big finale, which airs on Sky Atlantic and NOW TV on Monday, first at 2am (at the same time as it premieres in the US) and later at 9pm.

So, what truths do you think will be unveiled in episode six of The Undoing? Here are some of our favourite fan theories being shared on Twitter.

Sylvia is guilty – in more ways than one

Although Grace’s friend Sylvia has been acting pretty shady throughout the series, it’s been hard to predict how she might be involved in the whole debacle surrounding Elena’s death – that is, until now.

To start with, people are wondering whether Sylvia might be the second woman Jonathan admitted to having an affair with.

“Do we agree that this is the one other time Jonathan spoke about?” read one tweet. 

With that in mind, people are wondering whether Sylvia collaborated with Jonathan to kill Elena – by hiring a hit man.

“Next theory, [Jonathan] asked [Sylvia] for help (she was his one off) and she gave him the idea to ask [Franklin] for money to hire a hit man she knew,” one fan wrote.

And forgetting Jonathan entirely, some people think that Sylvia might be acting all on her own (and hid the hammer in Henry’s violin case) – either because she killed Elena, or because she’s assisting the prosecution to frame him. 

“She did it!” read one suggestion. “She was late to court because she went and put the hammer in Henry’s violin case at a time she knew no one would be home because they were all in court.”

“The way she was repeating what Grace said word for word on the phone after Grace had that call with Jonathan’s mom. She’s definitely helping the prosecution,” added another.

Franklin and Sylvia are in it together

When it comes to internet fan theories, Sylvia doesn’t get away easily. According to some people on Twitter, Sylvia and Franklin have been acting too mysteriously to not be involved somehow, especially because they seem to be pretty close.

“I just have a feeling that these two are up to something and could be behind all of this,” one fan wrote above a picture of Sylvia and Franklin in the courtroom. 

While another added: “I can’t stop thinking about how Sylvia waved at the prosecutor in the last episode when she walked into court. Their subtle cues and hers with Franklin have me on alert. They’ve all been up to something.”

Henry is concealing the truth

While it’s a more far-fetched theory than many of the others being shared on social media, we can’t rule it out – especially after last week’s hammer debacle and the fact that Jonathan was revealed to be a sociopath (something he could have passed on to Henry) in last week’s episode.

“I think it’s Henry,” wrote one fan. “When he said ‘sorry’ to Miguel, it seemed strange and he said it was one of the five Cs. Is he a sociopath who has learnt the ‘right’ things to say/do?”

While another added: “It was Henry. He didn’t even flinch in the courtroom when they showed that women’s face bashed open, and I don’t like how many times he said ‘sorry’ to Miguel when he ran into him at school.”

However, some fans were quick to note that Henry might not have acted alone if he was involved with Elena’s murder – instead, he might be hiding evidence for his Dad, who we know he worships.

“Why couldn’t the police find the hammer when they searched through the apartment – is Henry just trying to protect his Dad by hiding the hammer?” one theory read.

“Either Henry did it or he’s hiding that Jonathan really did it,” added another. 

And last, but by no means least, there’s a rather dark theory in the midst of all the debate going on on Twitter – that Miguel killed his mother, and Henry is helping him to cover it up.

“What if Elena’s son killed her because he saw her having sex with Jonathan,” one fan suggested. “And Henry is helping him cover it up. That’s why Miguel is always so weird around Henry.”

Another added: “I think Miguel walked in on Henry killing her or Henry walked in on Miguel killing her. They are covering for each other and Jonathan just showed up after the fact which is how his shoe prints were found there.”

Grace is hiding something

Although Grace is the protagonist of The Undoing, there’s still an overriding sense that we don’t know the full story about her actions in the lead up to and on the night of Elena’s murder. And for some people, that makes Grace a prime suspect.

“I feel it’s something to do with Grace’s childhood,” one fan suggested. “The titles show Grace as a child, lyrics are about a dream, and she walks at night. Maybe she is a sleepwalker and she killed Elena.”

“THIS!!” another fan replied. “Because you know how Henry mentioned in the restaurant that Grace said ‘if we survive this we can survive anything,’ and she said ‘I said that?’ I feel that was quite a significant.”

While another theory reads: “I think Elena was one of Grace’s patients. We wouldn’t know because Grace isn’t a suspect and she wouldn’t bring it up at their committee because of confidentiality. I think what appears to be her withholding is actually her patient privilege.”

And our all-time favourite theory…

“New #TheUndoing theory: one of Grace’s gorgeous coats came to life and did it.”

10/10 for imagination right there.

Episodes 1-5 of The Undoing are available to watch now on Now TV and Sky Go. Episode 6 of The Undoing airs Monday at 2am and 9pm on Sky Atlantic.      

Image: Sky