The novel begins with Lo Blacklock trapped in her bedroom as a burglar ransacks her apartment in London, England. She manages to escape but is traumatized, suffering from PTSD and prescribed antidepressants to help with the anxiety. Despite all this, she is determined not to miss a big work opportunity of covering a brand new Norwegian cruise aboard the luxury yacht, Aurora for the travel magazine Velocity.
Before leaving for the work assignment, Lo and her boyfriend Judah argue. She has been slow in moving forward with their relationship, while Judah has been patient and turned down a job opportunity in New York in order to stay with her in England.
On the first evening of the cruise, Lo asks her neighbor to borrow some mascara before the big welcome dinner. She’s surprised that her neighbor appears to be a young woman wearing ratty clothes. She seems flustered and keen on Lo leaving her alone, quickly shutting the door after giving her the mascara. Later on at the dinner, Lo meets with other journalist passengers aboard the ship, but does not see her neighbor. She notices how young the yacht’s owner Lord Richard Bullmer and how sick his wife Anne appears. Lo, still suffering from anxiety due to her burglary, copes by drinking too much at the dinner. Her ex-boyfriend and fellow travel journalist Ben Howard is one of the guests and he tries to make a pass at her before she knees him in the groin. She drunkenly heads back to her cabin and falls asleep.
Later that night, Lo is awoken by a scream and a loud splash. She runs out onto her balcony and sees a smear of blood on her neighbor’s balcony and what she believes is a woman swirling beneath the water. She hurries back inside and calls security but by the time Johann Nilsson arrives, the blood has disappeared. When Lo tells Nilsson about the woman she spoke with earlier, he tells her that cabin 10 has always been empty and that no one is staying in that room for this journey. He agrees to take Lo below deck and see if she recognizes the girl as any of the ship’s staff.
Despite not finding a match, Lo is determined to solve the mystery of her neighbor’s disappearance, believing she was murdered and thrown overboard. She discovers the mascara she borrowed from the girl is now missing and summons security again. This time, Nilsson begins to question her sanity, citing the amount she drank the previous night and her use of medication.
Later on, during a session at the ship’s spa, Lo falls asleep and awakens to discover that someone has written the message “Stop Digging” in steam on the mirror. Afterward, Lo meets with Lord Bullmer who has agreed to personally listen to her story of what happened the night before. He placates her but does not appear to believe her.
Back in her room, Lo finds that her balcony door can’t be locked and also discovers her cell phone is now missing in addition to the mascara. She decides that when the ship docks in Trondheim, Norway the next day, she will disembark and contact the police. In the middle of the night, there is a knock on her door and Lo is startled to find it is the woman from Cabin 10. The woman runs down the hall toward staff quarters and Lo follows her, but is knocked unconscious.
Lo awakens in a small, windowless room below deck. No one comes for her and she is trapped there suffering from anxiety and hunger. She also begins to feel the withdrawal from her medication. The woman starts to bring Lo food occasionally, with Lo finally attacking her with a food tray. The girl accidentally wipes her makeup off and appears to look just like Anne Bullmer. While alone, Lo starts to unravel the mystery of what has happened. She figures out that Anne is the woman that was killed and thrown overboard and the woman next door has been impersonating her since then.
Lo reveals this to the woman and she becomes scared and starves Lo for a couple days. Lo starts to hear the rest of the passengers disembarking and she realizes the cruise is over and everyone is leaving. When the woman finally returns, she has softened a bit and reveals her name is Carrie. She tells Lo that she is in love with Lord Bullmer and that they met while she was working as a waitress at a club. She thinks they’re in love but Lo tells Carrie that she has been used by Bullmer in order to kill Anne and secure her fortune. Carrie feels guilty about the plan and decides to help Lo escape.
Lo disguises herself as Carrie, while Carrie gets locked in the room. Lo is almost found by Bullmer but falls into the ocean instead. She swims her way to shore and seeks help at a hotel she finds there. She soon realizes that the hotel manager is a close friend of Bullmer’s and that he is also friends with the police there. She flees and hides in a barn, where an old Norwegian farmer discovers her and lets her use his phone. She calls Judah in England who raises the alarm. By that time, Bullmer is dead, with reports saying he committed suicide. Lo assumes he also killed Carrie.
Lo decides to move to New York with Judah and start over, taking a job in investigative journalism. After some months, the bodies of Bullmer and Anne are discovered and it’s shown that Bullmer was shot. Lo receives a money transfer from Carrie (in a code name), which confirms she has actually survived and most likely shot Bullmer.
