'It destroys me': Kristen Stewart explains the ending of Princess Diana movie 'Spencer' (spoilers!)

Spoiler alert! The following post contains details about the ending of "Spencer."

With the help of Colonel Sanders and a snappy pop song, "Spencer" gives Diana the fairy-tale ending she always deserved.

The Princess of Wales, played by Kristen Stewart in the Oscar-ready drama (in theaters now), died in a tragic car crash in 1997 when she was 36, after a high-speed chase by paparazzi through Paris. The beloved royal, who divorced Prince Charles a year prior, left behind two young sons, Prince William and Prince Harry (portrayed as boys by Jack Nielen and Freddie Spry, respectively, in the film).

Over the course of the claustrophobic movie, Diana repeatedly clashes with Charles and staff members at Sandringham House estate, where she's cooped up and closely surveilled over the Christmas holiday in 1991. But after an emotional heart-to-heart with her dresser (Sally Hawkins), Diana feels emboldened to take a stand against the oppressive royal family and buck tradition.

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She whisks her sons away from a Boxing Day hunt with their father, and the three of them blissfully leave the countryside manor in an open-top convertible, singing along to Mike and the Mechanics' "All I Need is a Miracle." The film cuts to an irked Charles having tea with the Queen (Stella Gonet) back at Sandringham. Meanwhile, Diana, William and Harry indulge in drive-thru Kentucky Fried Chicken. As her boys contently eat their lunch on a bench, Diana stares out wistfully over the River Thames before credits roll.

Stewart, 31, was just 7 years old when Diana died, but has "distinct visual memories of the flowers outside Buckingham Palace (on TV)," she says. "The emotion in the air was palpable."

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For her, it was important to give Diana a happy ending in the film, even just briefly.

"The story is about liberation and she did get out," Stewart says. "She did just decide one day to walk out the door that was not actually locked, and she lived the last couple years of her life in a much more inhabited and honest way. I think the movie ends on a light note because that is what happened. And then the last few frames of the movie, you can't help but bring the tragedy to that sort of enlightened moment."

"Spencer" director Pablo Larraín gave Stewart and the boys three songs to choose from for the ending. They landed on "All I Need is a Miracle," which was released in 1986 and reached No. 5 on the Billboard Hot 100 singles chart.

"There's just something about the swell of that song that feels like the end of a movie," Stewart says. "I love how the movie ends in this almost John Hughes type of way, where the female star runs off into the sunset and is stoked. And then it cuts back to the (royal) family, and they're all having this experience with the song that is so different from where she is headed.

"The movie ends triumphantly, but as soon as you realize that she doesn't get it – that it would take a miracle for her to actually still be with us – it destroys me. It just makes me cry, it's so sad."

Larraín recalls a special moment during shooting when one of the kids asked him what the miracle is that they're singing about.

"I said, 'It's right in front of you – it's your mom,' " Larraín says. "It's not only a beautiful piece of pop music that brings an uplifting moment by the end of the film, but it also speaks about their relationship. It's hard to see because it's small, but when (William and Harry) are in the car with Diana, they're pointing their fingers to her when they're singing. It's like a hidden map to the movie."

This article originally appeared on USA TODAY: 'Spencer' ending: Kristen Stewart says it 'makes me cry' (spoilers)