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Viewers are certain they know who the killer is in The Undoing 

Hugh Grant and Nicole Kidman star in the new crime thriller

Emmy Griffiths

Viewers have been gripped by The Undoing, and plenty are now convinced that they have worked out who the real killer is. 

READ: Hugh Grant has been watching Gogglebox stars watch The Undoing - and he has some thoughts

The six-part murder mystery, starring Hugh Grant and Nicole Kidman, follows a married couple who finds themselves in the centre of an investigation after the husband Jonathan's lover, Elena, is found brutally murdered. 

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WATCH: Episode four revealed another secret Jonathan has been hiding

So far, viewers have suspected every one of the main characters from Nicole's character Grace to the couple's son, Henry. However, following episode four, plenty of people took to Twitter to discuss their new theory, and we think they might be onto something!

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In episode four, Jonathan reluctantly admitted that he had been unfaithful to his wife on one other occasion, but didn't name the other woman. He later does a television interview where he suggests Elena was killed as a result of their affair, and by someone in a rage after finding out. 

While the show hints he is referring to Elena's husband Fernando, viewers are instead suggesting he is talking about Sylvia, Grace's fellow school mum friend, and that she was the other woman that Jonathan had an affair with. 

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Could Slyvia be the culprit?

One person wrote: "Twist coming. I think Sylvia had the sad affair with Jonathan and was jealous." Another added: "Just a hunch. I've been side eying Sylvia since around episode 2. I never felt she was authentic.   I think there are only 6 episodes, so 2 more to go."

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There are two episodes left of the six-part drama

A third person wrote: "Anyone notice the 'looks' that Sylvia and Jonathan were giving each other in the courtroom? Did they set up Grace to take the fall for something they did?" Another fan tweeted: "If not Jonathan, my money is on Sylvia (Lily Rabe), she was too keen to accuse the husband and since she was clearly aware of the affair and is Grace’s loyal friend, I could see her snap."

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