Two weeks after leaving for school, Sam finally calls Percy. He apologizes and says he loves and misses her, but he is evasive about the workshop, his dorm, and the people around him; when a girl named Jo interrupts to take him to a potluck, Percy becomes jealous and angry. Sam follows up with short, unsatisfying emails and texts, while Charlie notices how stuck Percy is and pushes her back into swimming. He meets her every morning on the dock, helps her train, and gives her a goal of swimming across the lake again before summer ends.

As August goes on, Percy becomes more anxious and possessive. She calls Sam constantly from the cottage landline, resents his lack of enthusiasm, and finally asks whether he took the workshop to get away from her. Sam insists he did not leave to escape Percy; he says he is trying to build a future for himself and is simply overwhelmed by the new environment. At home, Percy overhears hints that her parents are struggling with money, but she is too consumed by Sam to focus on it. Charlie remains her only real relief, and after Percy completes the lake swim, their banter turns more openly flirtatious; he praises her writing and suggests a drive-in movie to cheer her up.

The next crisis comes when Percy receives a long email from Sam. Sam admits he feels intimidated, lonely, and out of place at school, and he asks for boundaries: one scheduled phone call a week so they can each focus on their own futures. Percy reads the request as rejection and calls immediately, only to have Jo answer Sam's phone while he is drunk at a party. Devastated, Percy spends the night crying. The following evening, Charlie picks her up for the drive-in, notices she has been crying, and learns that Sam wants space. Percy asks Charlie not to dwell on it, and the two of them spend the night eating junk food, watching Rosemary's Baby, and talking easily on the drive home.

Back at the empty cottage, the tension between Percy and Charlie becomes unmistakable. Charlie teases her, tells her exactly how attractive he finds her when she gets worked up, and Percy kisses him because she desperately wants to feel wanted. They have sex in her bedroom, but in the morning Percy wakes beside Charlie, hears a voice that briefly reminds her of Sam, and realizes in horror that she has slept with the wrong brother. The shock triggers a panic attack. Percy throws Charlie out, then later screams at him when he returns to check on her; Charlie apologizes repeatedly and holds her until she calms down, and after that they stop speaking about what happened.

After Percy returns to Toronto, her parents tell her they are selling the cottage. Sam, meanwhile, grows warmer again: his emails become longer, he apologizes for his earlier distance, and he makes clear that the future he imagines still includes Percy. Percy postpones seeing him in person while deciding she has to confess the truth at Thanksgiving. Instead, when Sam comes home to help her pack up the cottage, he reunites with her lovingly and then drops to one knee, offering his grandmother's ring and asking her to marry him someday. Unable to accept the proposal or admit that she slept with Charlie, Percy lies, says they are too young and that she cannot trust his love, and asks for a break, using their bracelet oath to make the lie final.