Laura “Lo” Blacklock, a 32-year-old travel journalist, is trapped in the bedroom of her London flat one night while a burglar ransacks her belongings. After breaking her way out, she struggles with flashbacks and insomnia, but she’s determined not to miss her big break: covering the Aurora, a luxury yacht cruising the Norwegian fjords to see the northern lights, for the travel magazine Velocity. Before leaving for the cruise, she argues with her boyfriend, Judah, because she’s hesitant to move forward in their relationship, despite Judah waiting for months and turning down a New York job offer to stay with her.
By the time she’s welcomed aboard the Aurora, Lo is severely sleep-deprived and still suffering flashbacks to the burglary. She also uses alcohol to help her manage her anxiety. On the first evening of the cruise, Lo knocks on the door of the neighboring cabin to borrow some mascara. She’s greeted by a surprised young woman wearing a ratty Pink Floyd T-shirt. The woman seems flustered and abruptly shuts the door in her face after giving her the mascara.
During the first night’s networking dinner, Lo meets a number of fellow journalists, including her ex-boyfriend, Ben Howard, but she doesn’t see the girl again. She’s struck by the apparent youth of yacht owner Lord Richard Bullmer, as well as the sickly appearance of his wife, Anne Bullmer, who’s been fighting cancer.
Very late that night, after a drunken Lo has fallen asleep, she’s awakened by what she thinks is the sound of a scream, soon followed by a loud splash. She runs out onto the veranda and thinks she sees the shape of a woman’s hand swirling beneath the water, as well as a smear of blood on the neighboring veranda. By the time head of security Johann Nilsson arrives, however, the blood has disappeared. When Lo insists that she spoke to the girl in Cabin 10 earlier and that she must be in danger or dead, Nilsson tells her that cabin has always been empty.
The next morning, Nilsson takes Lo below deck so she can see for herself if she recognizes the missing girl among the ship’s staff. Though no one can give her any useful information, Lo is appalled by the contrast between the luxury upstairs and the dark, cramped staff quarters. Lo remains fixated on solving the mystery of the girl’s apparent murder, even when veteran journalist Tina West offers her a freelancing opportunity that could advance her career. Back in her cabin, Lo discovers that the girl’s mascara is now missing. She summons Nilsson again, but he questions her judgment in light of her alcohol consumption and her use of antidepressants. Lo realizes that if she wants to find out what really happened, she’s on her own.
During a mud wrap in the ship’s spa, Lo falls asleep, then wakes up to discover that someone’s traced a warning message in the steam on the bathroom mirror: STOP DIGGING.
That afternoon, Lo meets with Lord Bullmer personally to tell him what she’d heard last night, and he is disarmingly smooth and detached throughout the conversation. Later, Lo discovers that her veranda door can’t be secured—a clue as to how the mascara, and now her mobile phone, have been stolen from her cabin. Shaken, she settles on the couch for the night, waiting for the Aurora to arrive in Trondheim, Norway, the next morning so she can contact the police. However, there’s a startling knock on her door in the middle of the night—it’s the woman from Cabin 10, still alive. Lo chases her down the hall and through a staff door, then is attacked and knocked unconscious. She wakes up to find herself stuck in a cramped, windowless room in the bowels of the ship. Trapped for an indeterminate amount of time, Lo suffers from panic, hunger, and medication withdrawal. Though she’d clearly witnessed something she wasn’t meant to see, it wasn’t the murder of the mysterious girl, as she had first imagined. The girl had been sneaking into her cabin all along and left her the threatening message.
The girl brings Lo food occasionally, but always leaves before Lo can catch her. Finally, Lo attacks her with the sharp edge of a tray and bargains for access to her antidepressants. When the girl accidentally wipes off her penciled eyebrows, Lo suddenly recognizes her—she appears to be Anne Bullmer. Later, she pieces together the clues that, in fact, Anne is dead—killed the night the ship left England—and the girl has been impersonating her ever since. When Lo reveals that she’s caught on, the girl starves her for a couple of days in retaliation. In the meantime, Lo hears the rest of the passengers disembarking from the Aurora and nearly collapses from despair.
The next time she returns, however, the girl has softened toward Lo and even reveals her name—Carrie. Eventually, Carrie tells Lo how Bullmer had swept her off her feet while she worked as a waitress at his club, supporting herself as a struggling actress. The two had carried on a secret affair, with Carrie sometimes impersonating Anne in public. Lo figures out that Bullmer has intended to use Carrie all along—planning to kill Anne and exploit Carrie long enough to secure access to Anne’s fortune—but Carrie still maintains that she loves Richard and can’t betray him. Lo gets Carrie talking about the night she helped Bullmer throw Anne overboard—possibly alive—and the guilt that haunts her. She ultimately succeeds in forging a personal connection with Carrie, and Carrie hatches a plan whereby Lo can escape the ship.
Per Carrie’s plan, Lo disguises herself in Carrie’s clothes, locks Carrie in the room, and sneaks into the Bullmers’ suite to get some money Carrie’s left there. Bullmer suddenly returns, so Lo is forced to climb over the veranda barrier to reach the neighboring suite—but she falls into the ocean instead. She fights her way ashore and seeks help in a village hotel, but flees again when she realizes the hotel manager is Bullmer’s personal friend. She must outrun the police and take refuge in a remote barn until she’s rescued by an elderly Norwegian farmer and able to phone Judah back in England. Lo assumes that Carrie was killed by Bullmer. Bullmer is also reported to have committed suicide.
As Lo recovers from the traumatic events aboard the Aurora, she decides to move with Judah to New York and abandon her work at Velocity, rekindling her dream of becoming an investigative reporter. Two months later, after the bodies of Bullmer and Anne have been recovered from the North Sea, it’s determined that Bullmer was shot by someone, and Lo receives a transfer of 40,000 Swiss francs from Carrie—confirming that she’d survived after all.