The Undoing reveals Jonathan's shocking secret before episode 5 cliffhanger

Episode five – which airs tonight in the UK – saw Jonathan (Hugh Grant) confess a shocking secret he had kept from his wife Grace (Nicole Kidman) for their entire marriage: that as a teenager, he was responsible for the death of his four-year-old sister.

Jonathan appeared distressed as he told Grace that, decades earlier, he had been left in charge of his sister when she wandered onto a busy road and was hit by a car, claiming he had wrestled with guilt ever since.

And initially, Grace appeared sympathetic to her husband, who had previously told her a similar story but replaced his sister with the family dog in his version of events.

But in a crucial scene later in the episode, Jonathan's mother revealed to Grace via video chat that her son was not telling the full story, recalling how he had never shown any signs of guilt or remorse for the incident – essentially outing him as a sociopath in the process.

This certainly seems to point the finger for Elena's murder firmly back in the direction of Jonathan, after a few episodes in which the series seemed to have been hinting someone else was the real culprit.

That wasn't it for twists though. In one final scene, Grace accidentally uncovered some new evidence while exploring Henry's room: the murder weapon is enclosed in her son's violin case.

In true cliffhanger fashion, the episode ended before she was able to question Henry about how it got there, but the discovery opens up all sorts of new questions ahead of next week's finale.

Of course, it's entirely possible that Jonathan could have hidden the weapon in his son's violin case, but could it be that Henry himself actually committed the murder?

Earlier in the episode, he revealed how he had seen Jonathan kiss Elena outside the school, so he's certainly not without motive, although it would come as a huge shock to viewers if he was responsible.

At the very least though, it appears that Henry knows rather more about the crime than he has let on so far – setting us up for a must-watch finale next week.

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.