Plot
Guy and Prisca Cappa are going through a separation. They take their young children, Maddox and Trent, to a tropical resort as a final family vacation. At their hotel, Guy and Prisca are given complimentary drinks by employee Madrid, and the children befriend Idlib, the resort manager Nils's nephew. At night, Maddox and Trent overhear their parents arguing and play a message decoding game from Idlib. During the argument, it is revealed that Prisca has a benign slow-growing ovarian tumor.
The next morning, Nils invites the family to a secluded beach, joined by three additional parties: rapper Brendan "Mid-Sized Sedan" and his female companion, who already arrived; surgeon Charles, his elderly mother Agnes, his wife Chrystal, and their young daughter Kara; and Jarin and Patricia Carmichael, a close-knit husband and wife.
Trent discovers the corpse of Brendan's companion. Charles grows suspicious of Brendan, who reveals that he has been experiencing nosebleeds because of his hemophilia. After the children rapidly turn into teenagers and Agnes suddenly dies, the families conclude that the beach is aging them, with the occupants undergoing the equivalent of one year of aging every 30 minutes. At least one member of each family has an underlying medical condition. They discover that trying to leave will result in them blacking out and waking up back where they left.
In a fit of rage, Charles cuts Brendan with a pocketknife. The group watches as his injury quickly heals. Prisca's supposedly slow-growing ovarian tumor rapidly grows in size at a life-threatening rate. Charles successfully removes it surgically. Brendan discovers that his companion's body has fully decomposed in mere hours. Kara and Trent continue to age into their late teens, and after having sex with each other, Kara gets pregnant at a rapid pace. She gives birth, but the baby starves to death as the time rapidly passes.
The attempts to leave grow increasingly tense as Charles's worsening schizophrenia causes him to kill Brendan. As the day progresses, Jarin drowns while trying to swim for help, Kara falls to her death while trying to climb over the mountain, and Patricia suffers a fatal epileptic seizure. Guy's eyesight blurs, and Prisca suffers hearing loss. Trent and Maddox discover the notebook of a previous traveler filled with names of many people who died on the beach, along with indications of them being watched. At night, Charles attacks Guy and Prisca in a schizophrenic episode; Prisca runs as Charles continues to attack Guy.
Chrystal's hypocalcemia results in her bones repeatedly rupturing and improperly healing, gruesomely contorting her body and eventually killing her. Prisca returns and slashes Charles with a rusted knife, instigating a fatal blood infection that kills him. An elderly Guy and Prisca make amends before dying peacefully of natural causes, moments apart from each other.
With only the now-middle-aged Maddox and Trent remaining by the next morning, the pair construct a sandcastle. They revisit a secret message given to them by Idlib, which Trent deduces is connected to an underwater coral passage. Believing that the passage will allow them to leave the beach without losing consciousness, he and his sister start swimming through the coral. After they seemingly fail to emerge from the water when Maddox gets her swimsuit caught on a coral reef, a resort employee who is monitoring them reports that the entire group has died. His contact, Sidney, alludes to a previous incident where a guest almost escaped from the beach. Nils then announces that trial 73 has concluded.
It is revealed that the resort is a front for a research team from the pharmaceutical company Warren & Warren that was conducting underground clinical trials of new medical drugs, which are administered to guests with medical conditions by spiking their drinks. Because the beach naturally accelerates the lives of the subjects, the researchers have been able to complete the lifelong drug trials within a day.
Nils and Madrid prepare to lure a new group to the beach, but are interrupted by Trent and Maddox, who survived their underwater swim, using the notebook as evidence that they give to vacationing police officer Greg Mitchel. Discovering everyone listed in the notebook is a missing person, he informs his boss of the researchers' crimes. The researchers are mentioned to have been arrested, with subpoenas sent to the rest of Warren & Warren, as Greg plans to reunite Trent and Maddox with their aunt.