Nothing is as it seems on the superyacht Aurora Borealis — not even journalist Laura “Lo” Blacklock’s (Keira Knightley) assignment. But as The Woman in Cabin 10 quickly reveals, there’s more to this story than rich philanthropist Richard Bullmer’s (Guy Pearce) schmoozing on the high seas. 

“[Lo] thinks that this cruise story is going to be very easy, a puff piece, and it turns into this very dramatic tale,” Knightley tells Netflix. “She’s a very serious journalist who initially is incredibly embarrassed about going on a superyacht cruise and writing about these people, and then discovers that there’s more to the story than she thought.”

After a long night of drinking, Lo hears a struggle in the cabin next door to hers — and then sees a woman fall overboard. Lo’s fellow passengers meet her claims with skepticism if not hostility, and a race is on to discover the truth. “She starts off in this environment that is quite exciting and everybody’s very nice, and all of a sudden, it turns on her,” Knightley says. “And I think the fact of being trapped on this boat made it that much more claustrophobic, and hopefully more suspenseful, because you think, ‘OK, well if there is a killer on the boat, there’s no way off. You’re in the middle of the North Sea, where are you going to go?’ ”

You can follow along with Lo in The Woman in Cabin 10, now streaming on Netflix, and read on to dive behind the scenes of the film’s watery finale. 

Is there a woman in cabin 10? Or is Lo imagining things?

Lo arrives on the Aurora Borealis already on edge. Her last interview subject, a witness for a story about human trafficking, was murdered in front of her. Lo blames herself, and she’s having a hard time moving on. “She has that career-low moment that people have,” director and co-writer Simon Stone (The Daughter, The Dig) tells Netflix. “That’s it, it’s over for me, I can’t recover from this.”

Her friend and editor Rowan (Gugu Mbatha-Raw) encourages her to report a story on the Aurora to distract her from her problems. But Lo quickly gets drawn into a mystery instead. After spotting her photographer ex-boyfriend Ben (David Ajala) on the way to dinner, Lo ducks into cabin 10 to avoid him. There, she meets a mysterious woman (Gitte Witt) and thinks nothing of it — until the woman in cabin 10 falls overboard in the middle of the night.

Lo wakes the security team, led by Sigrid (Amanda Collin), who investigate. But everyone is accounted for, and the team tells Lo that the guest intended for cabin 10 canceled her trip two days before the cruise. Lo must have been mistaken … right?

But Lo is sure of what she saw — she knows there was a woman in cabin 10. Even so, Lo’s judgment is called into question; fellow passengers point out that she’d been drinking heavily the night before. Stone never intended audiences to distrust his protagonist, though. “It’s not an unreliable-narrator story,” he says. “I wanted to lean away from that. The audience doesn’t at any point question whether or not this person saw what they saw. You do agree completely with her, and you take it as fact. It’s her struggle against a conspiracy.”

So no, Lo isn’t imagining things. She even spots the mystery woman in a photograph Ben took at one of Richard’s parties. Then someone writes the word “STOP” on Lo’s spa-room mirror. And, most frighteningly, that same someone pushes Lo into the superyacht’s pool before covering it, trying to drown her. But everyone on board — including Richard — still seems determined to convince Lo that she’s imagining things. Why?  

Who killed the woman in cabin 10?

The key to this mystery lies in plain sight. Before Lo hears the woman fall overboard, she has a meeting with Richard Bullmer’s cancer-ridden wife, Anne (Lisa Loven Kongsli), whose illness the cruise is built around; the destination is a gala in Norway to raise money for cancer research. Anne invites Lo into the library and compliments her work, then asks the journalist for a favor: Go over her newly edited speech for the gala, in which she announces she’ll be giving away her entire fortune to charitable causes. “It’s about settling up,” she tells Lo. “Putting back in what you’ve taken out.” 

So when Lo sees Anne again a few days later and Anne gives her the cold shoulder, she’s shocked. And she should be: In a twist drawn from Ware’s novel, Anne has been swapped out for Carrie, the woman in cabin 10. It wasn’t Carrie who died but Anne, thrown overboard by Richard in a desperate attempt to reclaim the fortune she was about to give away. 

“I thought the conversation about the patriarchal assumption of being able to just replace women, dispensable, that you can dispose of them and move on, that’s at the core of the movie,” Stone says. “That is actually the linchpin of this entire film, this idea that you can just move on and replace someone.”

Carrie is a doppelgänger Richard discovered using his friend Lars Jensen’s (Christopher Rygh) facial recognition software. She was hired to impersonate Anne, edit her will, and give Richard full power over her estate — but Carrie didn’t sign up for murder. She kidnaps Lo and begs her to stop searching for answers, before Lo escapes the below-decks cabin where Carrie has hidden her. As for Carrie, her interaction with Lo has had an impact. While she’s signing the will, she hears Lo’s warning in her ears: Will Richard let her go free now?

Do two actors play Anne and Carrie?

Yes. Anne and Carrie are played by Lisa Loven Kongsli and Gitte Witt. The production considered having both roles played by a single actor, but they ultimately settled on casting two people. 

It took an exhaustive casting process to find the two actors. “Because they were to have little or no hair, then two head shapes could be very, very dissimilar, and you’d know at once that it wasn’t the same person,” makeup designer Jenny Shircore says. “So we spent a couple of days testing all these girls, putting bald caps on them just to see head shapes and what that does to the size of the face.”

Ultimately, the effect was accomplished with some simple cinematic sleight of hand. “Lisa, who plays Anne, really just has one main scene in the library,” costume designer Eimer Ní Mhaoldomhnaigh says. “So that’s helpful, in a sense. We’re not seeing the two of them all the time.” It’s Witt, as Carrie, who’s onscreen the most.

Kongsli and Witt are onscreen together at two points in the film: Once, during the flashback in which Anne stumbles on her husband and her doppelgänger — with tragic consequences — and once at Bullmer’s gala. “They had these big blurred pictures of the character Anne with the character Carrie [disguised] as Anne,” Shircore says, “and you couldn’t tell who was who. So I think it works.”

What happens to Lo?

While attempting to escape the yacht and crash the gala so she can expose Bullmer’s plan, tragedy strikes. Ben, not convinced by Bullmer’s story that Lo’s not feeling well, stays behind, finds her, and helps her escape Bullmer’s henchman, Dr. Mehta (Art Malik). But Ben doesn’t make it off the yacht alive: Mehta stabs him in the neck.

Lo makes her way to shore and to the gala, where she reads the revised speech Anne had planned to deliver before her death. Bullmer denies its validity, but Carrie, still in disguise as Anne, admits it’s the truth. A furious Bullmer takes his “wife” hostage with a steak knife and tries to escape, with Lo on his tail and Sigrid tracking him with a hunting rifle. Sigrid fires a round at Bullmer’s shoulder, but she can’t get a clean shot — until Lo sneaks up behind him and knocks him out.

In spite of everything, Lo and Carrie’s story ends happily. After days on the Aurora Borealis, they watch the real northern lights illuminate the sky. Anne’s fortune is donated to charity, and Lo returns to work and writes the story of the woman in cabin 10. An intern tells her she’s the reason she wanted to work at the magazine. “Anyone else would have made it all about him and what he did, but you managed to find the good in it,” she tells Lo. “It really surprised me.”

“Me too,” Lo replies. 

“Once Lo knows that something’s going on, she doesn’t take no for an answer,” Knightley says. “She just goes for it and goes for it and goes for it. I think that’s what I really liked about the character. She’s not embarrassed by it. Everybody thinks she’s crazy. She doesn’t care. She just keeps going.”

On Lo’s phone, there’s a video from Carrie, of her and her child, and an invitation to come visit. Another story, for another time.

The Woman in Cabin 10 is now streaming on Netflix.