The Way Way Back begins with a lively car ride featuring four individuals: Trent, played by Steve Carell, at the wheel, and beside him is his girlfriend Pam, portrayed by Toni Collette, a recently divorced mother. In the backseat, we find Trent's teenage daughter Steph (Zoe Levin), who is busy listening to music, while Pam's son Duncan (Liam James) occupies the rear-facing seat—nicknamed the Way Way Back, perfectly capturing the essence of the film's title.
Amidst the journey, Trent, in a bid to engage Duncan, prompts him to rate himself on a scale from 1 to 10. After some coaxing, Duncan reluctantly settles on a score of "6", but Trent quickly dismisses it, asserting he’s more of a "3" and that he needs to be more outgoing. As they arrive at Trent's beach house, they encounter their slightly tipsy neighbor Betty (Allison Janney), who is desperate for companionship. Betty has three children: an older son who later appears selling marijuana, a teenage daughter Susanna (AnnaSophia Robb), and a younger son named Peter.
As the family settles in, Steph eagerly grabs her swimsuit, and both she and Trent urge Duncan to join them at the beach, despite his reluctance. Ultimately, he goes along to satisfy Pam. At the beach, Duncan keeps his distance from Steph and her friends, who dismiss Susanna when she wishes to remain on the shore with her book. When Duncan inadvertently catches his sister's gaze, she teases him: > "Stop staring, perv." Feeling out of place, he soon departs.
On his way back to the beach house, Duncan meets Betty and Peter; Betty suggests they have a playdate, which Duncan brushes off. Once inside, he finds Trent's lively friends Joan (Amanda Peet) and Kip (Rob Corddry) creating a ruckus with Trent and Pam. Feeling out of his depth at dinner, Duncan remains quiet, lost amidst the noise of adults.
The next day, Duncan discovers a children's bike in the garage and pedals into town. He stumbles upon Owen (Sam Rockwell), who’s immersed in an old Pacman arcade game at a local pizzeria. Their casual chat ends abruptly as Owen's lunch break draws to a close. Returning home, Duncan faces yet another summer activity when he’s roped into going on Joan and Kip’s boat.
Wearing the lone mandatory life jacket, he joins Betty and Susanna for the boat ride, further complicating his already uncomfortable situation. After the boat outing, Duncan interacts with Susanna again, leading to an exchange marked by mild flirtation and awkwardness. Duncan humorously declares, > "It looks like it's going to be a hot summer," only for Susanna to dismiss him with, > "I thought we were done here."
The next transformational encounter comes when Duncan spends a day at the Water Wizz water park, where Owen, now his mentor, sets him up with a bathing suit rental through Lewis (Jim Rash) and introduces him to the fun-loving chaos of the place. They encounter Roddy (Nat Faxon), who runs the water slide and curiously treats Duncan like an artist observing patrons waiting to descend.
As the movie progresses, Duncan’s character evolves, gaining confidence while working at the water park. He finds joy in cheerfully dancing to entertain guests, even stealing moments of camaraderie with new friends and impressing some girls.
Through various misadventures, including a rainy day filled with a board game that sparks disputes, Duncan gradually realizes his burgeoning self-worth and charm. During a fourth of July clambake hosted by Betty, he witnesses Trent behaving unfaithfully with Joan, unraveling the complex dynamics of adult relationships.
One fateful night, during another party, tensions spike when Duncan witnesses his mother confronting Trent about his dishonesty, leading to a confrontation where he declares loudly in frustration, "He's sleeping with her! And you know it!" Infuriated, Pam storms off, catalyzing Duncan's own feelings of betrayal.
In trying to escape his worries and connect with Susanna, Duncan finds himself wrestling with embarrassing moments, from trying to kiss her only to retreat in shame, to attending a farewell party for Lewis, where they engage in a spontaneous water fight.
As summer comes to an end, Duncan learns about the truth behind his mother’s decision to leave with him and their shared struggles. Yet, amidst the chaos, Susanna appears to bid farewell, surprising him with a kiss and clarifying her previous rejection.
The final act unfolds with the group regaining the car positions from their initial journey. As they drive away, Duncan impulsively exits the vehicle, racing to the water park before they leave. He surprises Owen, who has been a guiding figure over the summer, as they engage in a spirited race on the water slide.
With the movie's closing moments echoing with the song "Power Hungry Animals," the bonds forged shine brightly as Pam shares a moment of understanding with Duncan, capping off a summer that has irrevocably impacted their lives.
