Hidden detail featuring Nicole Kidman in The Undoing opening credits

If you have watched the first episode of The Undoing, chances are you may have recognised the voice in the opening credits song.

HBO’s new mystery series The Undoing has debuted, well-equipped with an intense theme song.

The eerie opening number plays before the episode begins, featuring old clips of a little girl bouncing around with glittery toys and bubbles.

While she’s giggling and having fun, there are also quick cuts to darker images. One shot looks like she could be playing with blood.

The girl has a head full of orange ringlet curls – exactly the same hair as Grace Fraser (Nicole Kidman) in the series.

And after its debut, fans were quick to question who was singing the eerie cover of Dream a Little Dream of Me.

Moulin Rouge fans will recognise that voice – it’s actually the show’s star, Kidman, singing the ode.

Kidman recorded the song in lockdown, with the help of her musician husband Keith Urban.

The couple produced the track together in Urban’s studio.

“We were in lockdown. We were in Tennessee, in Nashville, and the director (Susanne Bier) sent me a text going, ‘Would you sing the title track, Dream a Little Dream, to the series?’” Kidman toldVulture .

“I was like, ‘What? No, I so don’t want to do that. My voice is not good,’ which is always my reaction, and she said, ‘No, no, I think you can sing it.’”

RELATED: Nicole Kidman says her kids ‘fired’ her

Nicole Kidman and Hugh Grant in The Undoing, streaming on Binge.

The Undoing, which hit streaming on Binge this week, follows Grace, and the gloomy circumstances that surround the publishing of her first book.

She’s a successful therapist in the Upper East Side of New York, living with her husband Jonathan (Hugh Grant) and son Henry (Noah Jupe).

But her tranquillity starts to unravel, however, as she discovers her husband might be responsible for a huge tragedy.

The Undoing streams new episodes Mondays on Binge

This article originally appeared on Decider and has been reproduced with permission