Spoiler alert: This story includes details of Netflix's limited-series mystery "The Perfect Couple," including whodunit.

The butler daughter-in-law did it.

The finale of Netflix’s six-part whodunit, “The Perfect Couple,” reveals who murdered maid of honor Merritt Monaco (Meghann Fahy) at the wedding of Merritt’s best friend Amelia Sacks (Eve Hewson) and Benji Winbury (Billy Howle). Their nuptials − at the Nantucket estate of silver-spoon-fed Tag Winbury (Liev Schreiber) and his wife, successful novelist Greer Garrison Winbury (Nicole Kidman) − were interrupted when Amelia found Merritt’s lifeless body floating near the beach.

The night before, Merritt confided in Amelia that her affair with Tag resulted in Merritt's pregnancy, coloring the Winburys − marketed as “the perfect couple” to sell Greer’s books − in suspicion. But it's Abby Winbury (Dakota Fanning), married to Tag and Greer’s son Thomas (Jack Reynor), who's the culprit. In contrast with Elin Hilderbrand’s 2018 novel that inspired the limited series, Abby’s role in Merritt’s death is much more intentional.

“We had to change (the ending) because we didn't want the audience to know what was going on,” Schreiber says. “We had to have that element of surprise. It's so important, not just to the book, but especially for the show.”

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Schreiber and Kidman reveal their thoughts on that shocking finale, and where Tag and Greer go from here.

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Who killed Merritt Monaco on ‘The Perfect Couple’?

In the TV adaptation, Abby drowns Merritt because Merritt is pregnant with Tag’s baby, and the newest Winbury heir would delay Thomas' access to his trust fund for 18 years. As Thomas warned Benji in the first episode, “They’re all in it for the money.”

Fanning relished portraying the killer, Schreiber says. “She loved it. She would hold it in our face every day." She is "the queen of vapid on this show. She's actually deeply intelligent. But she does vapid like you would not believe, and it's hilarious. It's so good.”

“And when you watch it again,” Kidman says, “and you see all the layers that lead to her, she's fantastic in it.”

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In the book, Abby slips a pill into the drink of Featherleigh, a woman with whom Thomas is having an affair. (In the series, her name is Isabel.) Abby hopes that a drugged Featherleigh won’t be able to have sex with her husband that night. But Merritt drinks the spiked beverage and drowns, drowsy from the medication.

Kidman, an executive producer, says Hilderbrand enthusiastically relinquished control. “Elin, she was like, ‘Take it and make great TV,’” she says. “’What I do is I write. I'm not taking control of this. … Please just make it your own but make it good.’ And so it was really freeing, I know for (director Susanne Bier) and I.”

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‘The Perfect Couple’ Season 2: Do Tag and Greer have another chapter?

Though the finale wraps up the murder mystery, Kidman feels there’s more to the story for Greer and Tag that could unfold in another season.

“Wait and see,” she teases when asked about the couple who seem to be going their separate ways in the finale after 29 years of marriage. “I think it's ripe” for more.

Greer, who revealed to her children that she met their father while working as an escort and not in an art gallery as she had claimed, is finally “taking control of what she wants,” Kidman says.

“When you see the final episode and she sort of says, ‘Listen, let's just say it like it was,’ that was really fun actually, to be able to do that, just rip the Band-Aid off,” she says. “I think there's still an enormous amount of history and an enormous amount of future.”

“They're taking a break for now, but who knows?” Schreiber adds. He’s also on board for more episodes, if Netflix agrees.

“You can have the best cast, you can have the best director, you can have tons of money, but you don't always get that synergy between people where they're making room for each other,” Schreiber says. “And this group of actors was really, really great at that. It was a lot of fun.”

‘The Perfect Couple’ opening credits song and dance

For the show’s introduction, the cast learned a choreographed dance performed on the beach to Meghan Trainor’s catchy “Criminals.”

“It took me two weeks to learn,” Schreiber says.

“Took me five minutes,” Kidman brags. But what Schreiber lacked in skill, he made up for with enthusiasm. “I was very excited," he says.

“And I wasn't,” Kidman says − because Greer wouldn’t have been, she explains. “It’s hard to glide on a beach," especially in her aqua column dress.

“Very, very hard on a beach in that dress,” she says. “But I was like, ‘We'll keep (the movements) small.’”