Fourteen-year-old Duncan from Albany, New York reluctantly goes on summer vacation to a beach house in a small seaside town near Cape Cod, Massachusetts with his mother, Pam; her wealthy boyfriend, Trent; and Trent’s stuck up daughter, Steph. Trent emotionally belittles Duncan, frequently making comments and gestures that are condescending and rude to him. Steph follows suit in attitude towards him and is spoiled by Trent. On the way to the beach house, Trent asks Duncan to rate himself on a scale of one to ten; Duncan rates himself a six while Trent tells him he thinks Duncan is a three. They arrive at the beach house and are greeted by the neighbors: the gregarious, hard-drinking Betty, her children Susanna and Peter, and married couple Kip and Joan. Later that evening, Duncan and Susanna have an awkward conversation from their adjacent porches.
Duncan discovers a small girl’s bicycle in the garage of the beach house and uses it to begin exploring the town. At a pizza restaurant, he runs into the staff of Water Wizz, the local water park. He meets Owen, who is playing Pac-Man, and eventually Owen takes Duncan under his wing and shows him around the park. Duncan meets the park’s colorful, rag-tag group of employees: Caitlin, Lewis, and Roddy. Several kids at the water park speak reverently of a legendary pass in the tube slide, wondering how it could have been done. Owen hires Duncan for odd jobs at Water Wizz.
Outside the park, Duncan is continually neglected by his mother, Pam, who indulges in drinking, staying out at night, and smoking marijuana with other adult vacationers. At a Fourth of July cookout, Susanna sees that he is upset about this and invites him to go hunting for ghost crabs with her and Peter, where she talks about her absent father and helps Duncan to open up. Later that night, Duncan witnesses Trent and Joan kissing by the side of the house, but does not reveal what he saw.
Pam begins to suspect Trent and Joan are having an affair, but Trent convinces her nothing is going on. Later, Duncan confronts Pam in front of friends and neighbors and tells her to face up to Trent’s affair and get rid of him. Trent in retaliation tells Duncan his divorced father does not want him. Duncan stalks away. Susanna follows him and comforts Duncan out on the beach. Duncan attempts to kiss Susanna, but she moves away, which makes him become even more upset. Accompanied by Peter, Duncan sneaks away to Water Wizz where Owen is throwing a going away party for Lewis.
After spending all night with his friends at Water Wizz, Duncan is still at the park the next morning, refusing to leave. Owen confronts him and asks him why he does not want to go home. Duncan opens up to Owen about his relationship with Trent and how the water park is the only place where he feels happy and accepted. Owen sympathizes with Duncan’s problems.
When Duncan arrives back at the beach house, Pam tells him they are leaving with Trent and Steph. Betty and her kids arrive to say their goodbyes. Susanna finally kisses Duncan. When Trent stops for gas on their way out of town, Duncan jumps out of the station wagon and runs to Water Wizz, followed by his mother, then Trent and Steph. Duncan tells Owen and the other employees that he has to leave and tells Owen to follow him. He takes Owen to the Devil’s Peak slide, and Duncan becomes the first person to ever pass someone in the water slide while the rest of the park watches. After finally introducing Owen to his mother, Duncan says goodbye to everyone at the park. Owen tells Pam she has a great kid, and introduces himself to Trent as “a good friend of the three”. Trent, Steph, Pam and Duncan regroup in the car, where Pam finally stands up for herself as they head out of town. Pam climbs to the way, way back of the car where Duncan is sitting and they share a smile as Trent’s protests are heard in the background.