Laura 'Lo' Blacklock, Keira Knightley, is a tenacious investigative journalist trying to recover from a traumatic confrontation when she receives an exclusive invitation. Terminally ill billionaire Anne Bullmer invites her aboard a dazzling luxury superyacht, where she and her husband, Richard Bullmer — a man with a meticulous plan and a dangerous appetite for control — host a gathering of their wealthiest guests to launch a new charitable foundation. The voyage to Norway promises access, influence, and a chance for Lo to craft a profile that could redefine her career, but beneath the glittering surface tension simmers, and Lo senses that something heavy and complicated is about to surface.
That first night, Lo slips into Cabin 10 to dodge her ex-boyfriend Ben, David Ajala, and encounters a mysterious blonde woman, a sign that the trip will not be as quiet as it seems. After dinner, Anne privately confides that she has stopped taking her medication and hands Lo a copy of the speech she intends to deliver at the gala—an announcement that would reveal a monumental shift in her fortune. When a woman screams in the dark later that night, Lo discovers a bloody handprint by Cabin 10 and witnesses someone go overboard, but when the passengers are accounted for, the handprint vanishes and no guest is listed in Cabin 10. The sense of danger grows as Lo dives deeper, determined to uncover the truth behind the incident.
As the voyage continues, Lo notices a striking blonde woman from a party captured in photographs taken by Ben, a person Adam [an unnamed passenger] claims not to remember. At the spa, a cryptic message—“STOP”—appears on Lo’s steamy shower door, and she learns the ship’s security cameras have been deactivated for privacy. Sneaking into Cabin 10 again, Lo finds blonde hair in the sink and stashes the evidence away in her own cabin. She confides in Ben, but signs point to someone having searched her room, including a mysterious button on the floor. The danger intensifies when Lo is pushed from the upper deck into the pool and teeters on drowning; the hair and button vanish again after her coat is laundered, and she notices a missing button on Dr. Robert Mehta’s coat, a doctor who plays a key role in the tangled web around Anne and her fortune. The crew refuse to answer questions, and the guests suspend disbelief, leaving Lo increasingly isolated in her pursuit of the truth.
In a tense turn, Carrie, the woman from Cabin 10, reveals a chilling truth: Richard arranged for Carrie to impersonate Anne so she could sign over the fortune to him. After a private talk with Lo on the first evening, Anne discovers Richard kissing Carrie, and in a ruthless act, he murders Anne and dumps her body overboard. Carrie, desperate to secure her family’s future, fears Richard’s response even as Lo warns that the plan could end with Carrie dead as well. Carrie overhears Richard ordering Dr. Mehta to locate and kill Lo, and once Ben returns to the ship as the other guests disembark in Norway, the noose tightens.
Cornered by the assembled crew and the ship’s compromised captain, Lo is left with few options. Ben reappears, only to be jabbed with a lethal injection, and Lo makes a break for shore by jumping into freezing waters. Mehta falsely proclaims that Lo could not have survived, while Carrie—disguised as Anne—signs a revised will that raises immediate suspicion among Anne’s legal counsel and her bodyguard, Sigrid Nilssen (the bodyguard’s name appears in the plot but is not tied to a listed actor slug in the cast). Richard, realizing he is about to be cut off from his prize, readies a desperate act, but Lo’s expertise and Carrie’s reluctant cooperation tilt the balance in the unraveling plot.
In a dramatic gala moment, Lo interrupts Richard in front of all the guests and, with Carrie’s quiet support, reads Anne’s previously prepared speech that reveals Richard’s bid to cut himself off from the fortune. With the revelation hanging in the air, Richard lunges to seize Carrie, declaring her not to be Anne and taking her hostage at knifepoint as the yacht’s dock approaches. Nilssen retrieves a rifle and wounds Richard in the shoulder, and Lo uses a nearby crank to strike the fatal blow that ends Richard’s schemes for good. Carrie's willingness to reveal the truth produces a cascade of consequences, and the truth about the murders is brought to light.
The exposé Lo crafts — published with care and persistence — leads to the arrest of Dr. Mehta and the captain in connection with Ben’s and Anne’s murders, while Carrie returns home to her daughter, free from Richard’s reach but forever marked by the peril she endured. Lo’s work not only clears Anne’s name but also exposes the greed and deceit that festered beneath the yacht’s opulent surface, offering a hard-earned sense of closure for a journalist who faced danger to tell the truth.
