The Undoing teases new killer theory in episode 4 cliffhanger

The episode sees Jonathan pay a visit to Elena’s widower Fernando, desperate to claim his innocence in Elena's murder (of which he is the number one suspect) with his lawyer Hayley Fitzgerald not looking kindly upon the meeting.

Fitzgerald then sets up an interview for her client with retired news anchor Connie Chung, to be broadcast at a primetime slot on US television.

It's during this interview that Jonathan drops the bombshell – telling Connie that he believes he knows the true identity of the murderer.

Unfortunately, the episode ends before he is able to reveal that identity, and so fans will have to wait patiently until next week before hearing this intriguing new theory.

Several theories have been put forward by fans regarding who could have been responsible for the murder, with Fernando, Grace and even Grace's father Franklin all having been suggested as the possible perpetrator.

The series has slowly dialled up the mystery since its opening episode, when Elena was gruesomely murdered before Jonathan mysteriously went missing – fuelling suspicions that he was the culprit.

These suspicions were furthered in subsequent weeks when it emerged Jonathan had been having an affair with Elena, whose son he had been treating, and furthermore that he was the real father of Elena's recently born second child.

However, it seems that the real answer as to what happened is rather more complicated and that someone other than Jonathan was actually responsible for the brutal killing.

The series airs on Sky Atlantic in the UK and The Undoing cast includes A-listers like Grant alongside Nicole Kidman, who also sings The Undoing theme song, Noah Jupe and Donald Sutherland.

The Undoing airs on Sky Atlantic at 9pm on Mondays and is also available on NOW TV. Sign up for a seven-day free trial of NOW TV's Entertainment Pass, autorenews at £9.99 a month unless cancelled. While you’re waiting, visit our TV Guide to see what’s on tonight, or check out our guide to new TV shows 2020 to find out what’s airing this autumn and beyond.