Married couple Guy and Prisca Cappa are going through a divorce. To avoid saddening their young children Maddox and Trent, the couple takes them on a tropical resort as a final family vacation. At their hotel, Guy and Prisca are given complimentary drinks and the children befriend the resort manager’s nephew, Idlib.
At night, Maddox and Trent overhear their parents arguing and play a message decoding game with Idlib. The following morning, the manager invites the family to a secluded beach where three additional parties are present; rapper “Mid-Sized Sedan”, and a female companion; surgeon Charles, his wife Chrystal, their young daughter Kara, and Charles’s mother Agnes; and Jarin and Patricia Carmichael, a close-knit husband and wife.
Trent discovers the dead body of Mid-Sized Sedan’s companion. Charles grows suspicious of Mid-Sized Sedan, who reveals that he has been experiencing nosebleeds because of his affliction with haemophilia.
After Agnes suddenly dies, the three families conclude that the beach is rapidly aging them, with the occupants undergoing the equivalent of a year of aging every 30 minutes. At least one member of each family has an underlying medical condition and trying to leave will result in them blacking out. In a fit of rage, Charles cuts Mid-Sized Sedan with a switchblade and the group watches as his injury quickly heals. After Prisca develops a stomach tumor, Charles performs a successful surgery to remove it. Mid-Sized Sedan discovers that the body of his companion has decomposed in a matter of hours.
All three children have now aged into adulthood. A bond between the maturing Kara and Trent leads to Kara rapidly giving birth, only for the baby to quickly die from the beach’s effects. Trent and Maddox discover the notebook of a previous traveler, along with indications of them being watched. The attempts to leave grow tenser when Charles’s worsening schizophrenia causes him to kill Mid-Sized Sedan. Jarin drowns and Kara falls to her death as they look for a way off, Patricia suffers a fatal epileptic seizure and Chrystal’s hypocalcemia results in her bones rupturing, killing her. Charles attacks Guy at night in a schizophrenic episode, but Prisca slashes him with a rusted knife, instigating a fatal blood infection that kills him in seconds. An elderly Guy and Prisca make amends before dying moments apart from each other.