The Kapa family - parents Guy (Gael Garcia Bernal) and Prisca (Vicky Krieps) and their kids eight-year-old Maddox (Alexa Swinton) and six-year-old Trent (Nolan River) - arrive at a fancy resort for a family vacation. After settling into their rooms, Trent befriends a young boy named Idlib (Kailen Jude), and they do activities like ask other resort patrons what their names and jobs are, and Idlib shows Trent a special code to decipher messages. Meanwhile, Guy and Prisca discuss that this is just one last trip they wanted to give the kids before they divorce, which is made worse because Prisca is suffering from a stomach tumor.At breakfast, a middle-aged doctor named Charles (Rufus Sewell) is sitting with his younger vain wife Chrystal (Abbey Lee), daughter Kara (Kyle Bailey), and his mother Agnes (Kathleen Chalfant). Nearby, a woman named Patricia (Nikki Amuka-Bird) is having an epileptic seizure, which Charles helps with while her husband Jarin (Ken Leung) also tries to help. Trent also spots Idlib, as they made plans to hang out again that day, but the resort manager (Gustaf Hammarsten) makes him leave the area.The Kapas, plus Charles and his family, all board a van that takes them to a private beach. There, Maddox recognizes a rapper, Mid-Size Sedan/real name Brendan Smith (Aaron Pierre), who was earlier seen with a female companion who went skinny-dipping. The kids all play together while the adults lounge on the sand. The girls find old dolls buried in the sand, left behind by other kids. Trent swims by a cave when he discovers the body of Brendan's companion, apparently having drowned. The adults examine the body, while Brendan is suffering from nosebleeds and is equally stunned at the discovery. Charles suspects Brendan of foul play, though he maintains that the woman only went for a swim and never came back. Not long after, Agnes begins to fall short of breath before she also dies.Jarin and Patricia arrive at the beach as well, but the people who brought them are already gone. The kids also start to experience something weird. Brendan explains to the adults that the woman he was with had multiple sclerosis while he has hemophilia, which explains his constant nosebleeds. Jarin and Patricia talk to Trent and Maddox, who have suddenly aged up into pre-teenagers (Trent now played by Luca Faustino Rodriguez, Maddox played by Thomasin McKenzie), along with Kara (now played by Mikayla Fisher).Several attempts are made to leave the beach, by going through the rocks that close off the beach from the outside, but all the adults that attempt to pass through feel a pounding in their heads before they black out and end up back on the beach. They also notice unusual things, like when Charles attacks Brendan with a knife, the slash mark heals within seconds. Not long after, Brendan finds that the dead body of the woman he was with has now fully decomposed to nothing but bones within hours, making them realize that there is something time-related occurring on the beach. Suddenly, the tumor in Prisca's stomach starts to cause her pain and distress. The adults try to help her, with them feeling around for the tumor at first and describing it as the size of a golf ball. They proceed to perform a surgery to remove the tumor, which has now grown to the size of a cantaloupe. The cut in Prisca's stomach heals rapidly after the procedure.Trent and Kara age up even more into teenagers/young adults (Trent now played by Alex Wolff, Kara played by Eliza Scanlen). What's more is that Kara has somehow become five months pregnant within a matter of minutes (after apparently having sex with Trent minutes earlier. The adults prepare themselves for the birth, which happens rather quickly, but the baby dies after a minute due to lack of attention. The other adults start to show more visible signs of aging, but Chrystal takes the process much harder than the others.Charles starts to lose his mind to schizophrenia, leading him to stab Brendan to death. The others take his weapon away while trying to come up with an alternate plan of escape. Knowing they can't go through the rocks, Jarin attempts to swim to find a way out. While this happens, Trent and Maddox come across a notebook left by a previous visitor, with names and medical conditions written down. They also notice what looks like people in the distance observing them.Maddox talks to Prisca about whether or not she and Guy are splitting up. Prisca admits that she was seeing another man. Although Prisca tries to explain herself, Maddox is angry with her. She goes out to the beach and attempts to be strong and find a way out herself, but she finds Jarin's body, apparently having gone unconscious and drowned. Upon seeing his body, Patricia suffers a fatal seizure. Kara then desperately tries to find a way out and starts climbing up the rocks to try and make it to the top, but before she can make it high enough, she too blacks out and falls to her death, leaving Trent devastated.It is now nighttime. Charles has become fully unhinged while Chrystal is also losing her mind at her rapid aging and declining physical state (she has grown a calcium hump on her back). Guy is also losing his eyesight, and Prisca is noticeably grayer. Guy and Prisca talk about the other man in Prisca's life, with Guy admitting he knew about him and wished that he did more to keep their marriage working, but he also notes that the other guy is a loser and that she deserves someone better. Charles then starts to attack Guy and Prisca, while Trent and Maddox are attacked by Chrystal when they try to look at her. She follows them into the cave and her body becomes twisted and contorted to horrible angles before she dies there. Prisca then slashes Charles with a rusty knife, causing an infection that rapidly spreads through his body and kills him.The Kapas then gather around the campfire and the beach. The now elderly Guy is having a hard time speaking and remembering things, but he essentially admits to Prisca that he still loves her just before he dies. Prisca then stands up for a moment but collapses and dies by Guy's side while both kids mourn their parents.In the morning, Trent and Maddox are now in their 50's (Trent played by Emun Elliott, Maddox played by Embeth Davidtz). Trent brings up a code that Idlib gave him, which he deciphered and figured out means "My uncle doesn't like the coral". The siblings spot a coral reef nearby and realize that it must be their way out. The two swim out there, but Maddox's clothing gets stuck on the coral. When the two don't resurface, Theo, the van driver that brought the families to the beach (director M. Night Shyamalan) reports to his bosses, from his observation post on a cliff overlooking the beach, that all members of "Trial 73" have perished.When Theo drives back to the resort and to a lab area, it is then revealed that the resort is a front for a medical research team named Warren & Warren that lures people with medical conditions to the beach in order to perform clinical trials for life-saving drugs, given to them in the cocktails served upon arriving. They know that the supernatural beach (possibly powered by the mystical coral) speeds up the aging process, so they reduce the trials to a whole day for better results. They believe they cured Patricia's epilepsy since she didn't have a seizure for eight hours, which is equivalent to about 16 years on the beach.The resort manager and others attempt to poach their latest victims until they are stopped by Trent and Maddox, who managed to free themselves from the coral. Trent finds a man he spoke to when he was still a kid, knowing the man is a police officer. He hands the man the journal found on the beach, which details the names of many missing people and their medical conditions. Idlib comes out and recognizes Trent even as an adult. Trent and Maddox reveal the truth to everyone while all the resort staff realizes they are screwed.Authorities come to arrest all the resort employees, while in the final scene, the officer takes Trent and Maddox by helicopter away from the island to be with their aunt, who is not ready to see that her niece and nephew are full-grown adults.