The Undoing is the most glamorous and addictive TV thriller of 2020.

Nicole Kidman and Hugh Grant star as the wealthy and privileged New York couple Grace and Jonathan, a psychologist and child cancer specialist doctor. The seemingly easy-going couple have their lives turned upside down when Elena (Matilda De Angelis), a mum at their son's school, suddenly befriends Grace.

*Warning major spoilers ahead*

Hugh Grant and Nicole Kidman pose in artwork for The Undoing

At the end of episode one, Elena is brutally murdered, shortly after attending an opulent school fundraising event.

However, this is only a minor twist in comparison to what lays in store for the rest of the series. It’s been so surprising, we’ve even forgotten to stop gawping at Kidman’s stunning big coat collection and marvelling over the opening theme tune the actress sings on.

First of all it was revealed that Jonathan isn’t the dashing, lovable, bumbling man that we presumed him to be – Hugh Grant is very much playing against type this time. He disappears suddenly and he’s quickly the police’s prime suspect in the case.

Week after week the show has dropped classic bombshell moments that have left our heads spinning as we attempt to figure out who killed Elena.

Everyone from Grace’s shifty super-rich father Franklin, played by the incomparable Donald Sutherland, to her son Henry (Noah Jupe), has become a suspect for eagle-eyed viewers.

We’ve rounded up all the best fan theories ahead of the final episodes in the series…

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Who killed Elena in The Undoing?

Jonathan did it…

Hugh Grant’s doctor may have briefly fooled us in the opening episode that he was a charming doctor, who specialised in saving the lives of child cancer patients, but by the end of episode four our view of him couldn’t have stooped much lower.

Not only did he have an affair and a baby with Elena, his performance during a live TV interview showcased his most calculating side as he attempted to position himself as a victim in the case.

Hugh Grant's Jonathan glances over his shoulder and smiles

And the story about Jonathan’s childhood pet dog being killed could have been a hint at his true dark side; it turns out that it wasn't his dog that was killed, but his four-year-old sister - she let herself outside and was hit by a car when Jonathan was supposed to be looking after her. Furthermore, his mother told Grace that Jonathan never apologised for the death or even grieved. Was this an early glimpse at Jonathan’s psychopath tendencies?

Grace did it…

The haunting opening credits involve a young girl playing with bubbles in a garden to Nicole Kidman singing Dream a Little Dream of Me. They play into the theory that the show's final twist will be that Grace is the real killer.

Grace and Elena share an intimate moment in The Undoing

Do all of the flashbacks and vivid dark dreams from Grace come from her own actions that she’s blanked out or covered up?

She might have known about Jonathan’s affair and hunted down Elena. Fans have also pointed out that Grace spoke frequently to some of her clients about the nature of affairs and people wanting to get caught. Was she speaking from her own experience?

And an even wilder theory is that she was also having an affair with Elena. Perhaps her actions come from jealousy that she wasn’t the only person in her marriage that Elena was interested in.

And those late night walks around the streets of New York have to mean something…

Franklin killed Elena…

Grace’s father has been shiftily moving his eyebrows and glaring at paintings for far too long not to be up to something rather terrible.

From his unusual confession about his numerous affairs to the big wodge of cash he admitted giving Jonathan in secret, there are plenty of reasons to suspect the wealthy grandfather is up to his neck in it.

Could he have killed Elena in anger after learning about the affair? How far would he go to protect his daughter and grandson?

Franklin, played by Donald Sutherland, in The Undoing

One fan theory is that Elena is another daughter from one of his affairs – fans have spotted a blurred picture of a man with a very faint resemblance to Franklin in the back of a shot. Could this be another reason why he would want to silence the young artist?

Our favourite Franklin theory was that he’s actually dead and we’re only seeing him in the imagination of Grace. That would explain why his interactions with Henry are non-existent. Sadly, the theory fell flat on its face when you throw in the bail money and his interview from the police. But it was a nice idea while it lasted.

Henry killed Elena

Henry takes a call while the police search the family home in The Undoing

Henry became a suspect when Grace discovered a hammer - which could have been the murder weapon - in his violin case in his wardrobe. Pair that with the revelation that he knew about his father's affair, and all of a sudden Henry 'accidentally' bumping into Elena's son at school feels much less innocent.

With Jonathan's remorseless behaviour as a child, could it be a case of 'like father, like son'?

Sylvia killed Elena...

Lily Rabe in The Undoing

The lawyer, mother and friend of Grace is a possible outside bet as the killer.

Jonathan has admitted to one other affair, but didn’t reveal her identity. But his lingering looks at Sylvia (Lily Rabe) in the courtroom offered a clue.

Could she be involved in a long-term romance with Jonathan and acted in rage at the discovery she wasn’t the only extra woman in his life?

Watch The Undoing on Mondays on Sky Atlantic with NOW TV.

Images: 2020 Home Box Office, Inc. All rights reserved. HBO® and all related programs are the property of Home Box Office, Inc

Watch all the TV you love on BT TV

Catch your favourite shows and all the best sport with our flexible TV packages.  Entertainment packs include Netflix and Sky channels from NOW.