Mark is begrudgingly vacationing with his family - his wife, Rachel, and their two sons, Brian and Tyler - by the beach in a hotel called Ocean's Song. They are joined by Rachel's twin, Geoff, and his family - Geoff's wife, Sasha, and their two children.

Later that afternoon, the cousins are all playing at the beach, and Mark is unhappy because he does not really like Geoff or know his in-laws very well. He suggests going out for dinner, but Sasha protests by saying that her toddler-aged daughter, Harper, is getting tired keeping up with Mark's older sons. Mark remembers Rachel told him to be considerate of her brother's budget and offers to pay for the meal.

On the way to dinner, Geoff is loud and takes command, which upsets Mark because Mark is the one who suggested eating out in the first place. He entertains the kids with a "shell game" where he asks them to guess which of three moist towelettes his beer cap is under but then hides the cap in his hand. Geoff becomes loud again, and Mark is annoyed.

Sasha and Mark share a tenuous bond when they talk about how it's weird Geoff and Rachel are always talking. Mark reconsiders how Rachel always knows when Geoff will call and is so quiet after their hour-long conversations. He has always thought of it as a necessary "twin thing", but Sarah seems more critical.

Mark and Geoff prepare breakfast for the kids in the morning, and Geoff explains he wants to go back to school to learn how to code his app. He speaks about political correctness, what it means to be a man, and how society needs him to be in charge of his app idea. Mark remembers that Rachel told him that Geoff was never the same after college and how Mark's father-in-law said Geoff's not allowed to vote.

Later the two families go to the beach, and Rachel tells Mark that Geoff doesn't trust anyone to control his app so he wants to do everything himself. She says that Geoff is brilliant but can't deal with real work, so Sasha pays for everything and is always making sure their family is ok. Mark has grown more attached to his niece, Harper, and defends her against Rachel's critique of the frilly outfits Sasha always dresses Harper in. When they're back at the vacation house, it begins to hail, and Geoff goes outside to prove it's not dangerous. The kids are spooked, Mark is annoyed with Geoff's bravado, and later that night, Rachel seems very worried about her brother.

In the morning, Geoff asks Mark to take care of the kids, and Mark realizes it's so that he and Sasha can have sex. At the pool later that day, Geoff asks Mark if he thinks about the future. Geoff says the fate of the white race is uncertain, and his app is a way forward. He blames the media for the current political situation, and Mark is ashamed that he doesn't push back on Geoff's ideas more. Geoff's comments and Mark's silence only agitate his own insecurities with his increasing age and his masculinity. Mark goes to change Geoff's baby's diaper, and the lifeguard walks in. The lifeguard offers to hold the baby while Mark pees, but he struggles because he gets hard and is jealous of the lifeguard's youthful beauty and heroic job.

They go out for dinner on the last night, and Mark holds Harper in his lap. She asks him to draw her something beautiful. The next time Mark sees Harper, she will be a teenager, and they will be in the parking lot of a motel in their church clothes. Sasha will be angry, Rachel will be resigned, Mark will be guilty, and the baby will be a husky adolescent. In the present, Mark agrees to draw Harper something beautiful even though he doesn't have anything to draw with.